Could MMRPGs be used to test economic theories?

I second the Eve Online suggestion as an example of what you are looking for. Relevant features of the game are: 

• Wealth is the measure of player progress, and the purpose of player activity. This is because experience accumulates with time rather than kills or quests, and you simply buy the necessary skills when you have the prerequisites. Further, your ship is destroyed it is gone - you have to buy a new one. Though wrecks can be salvaged for parts.
• Players participate in almost every level of the economy: they manufacture most of the ships and many of the weapons; they extract most of the raw materials used to build those ships; etc.
• There is an explicit sector of traders, of both the merchant and speculative variety. The game has explicit transaction costs, and the skills in this area are centered around reducing those transaction costs.
• The game deals with hard-to-reconcile social values which compete with economic ones (PvP is a wealth-destroying activity, and is called "content" because it is usually more entertaining than mining space rocks).
• Players responding to economic incentives has several times unbalanced the game in such a way the game company needs to intervene. For example the most recent changes are being imposed to make raw materials more rare, because player mining got so efficient that ship prices plummeted across the board, resulting in wars among player alliances lasting forever because no one would ever run out of hulls.
• The company, CCP, keeps at least one full time economist on staff, whose job is to help model the economy of the game.

Based on this example, the things a game needs to be able to show complex economic behavior are:

• Players need to be able to both add and destroy value, which is to say there needs to be player driven production as well as material losses.
• Players need to be able to exchange value, which means being able to buy and sell almost all things within the game.
• The parts of the game need to interact, so that changes in value in one segment of the game can affect completely separate parts of the game.

I've heard of quite a bit of research using Eve Online -- a quick Google Scholar search suggests 3,870 results at the moment.

Would it be possible to create an MMRPG with an accurate enough model of the economy to test economic theories whilst still being fun to play?

Mu. Take a game that's fun to play, and improve the accuracy of its econ model, rather than the other way around.

I suspect that starting with a desire to test economies and building a game around it may not be the order of operations that would get the best balance of quantity and quality for data. If a game feels like a chore to players, they're less likely to engage for as much time over a long term as they do in games which are described as "addictive". Instead, I would start with a game that's already mastered the player addiction loop, or even rip off the gameplay of something successful, and change as few things as possible to allow it to emit the kind of data I wanted.

What would the plot of your economic MMRPG be?

I would avoid games with a single plot for all players, because I think open worlds offer a more accurate simulation of the choices that people in real economies face. Plots with warring factions could emulate the dynamics between countries in the real world, but plots with individual NPC bosses that players team up to take down seem to make collaboration easier than it is in reality. I think it'd work only in as much as that type of boss is analogous to natural resources, or possibly big bosses could serve a function similar to scientific breakthroughs if the game lacks another way to unlock new mechanics.

How would it be funded?

If one used a sufficiently addictive game, players would pay to play it.

What types of inaccuracies would you expect results from theories tested that way to contain?

I would expect inaccuracies anywhere that I hadn't scrutinized for accuracy. In particular, economic changes resulting from exploiting bugs in the game itself may show up without real-world analogues. This could potentially be managed by introducing intentional and more-exploitable bugs that would be more realistic than any accidental ones, such as making it easy-ish to illegally print unlimited money for the tradeoff that the money could be detected as forgeries with sufficient scrutiny.

The plot of my economic MMRPG would be humans in the far-future mining resources to build spaceships that they use to mine more resources and/or attack each other.

You might find it interesting that EVE Online hires economists to keep their in-game economy healthy, which among other things includes setting transaction taxes at a level that keeps inflation under control. You can see their March 2021 economic report here.

I'm having trouble finding a good article about this, but here's one.

MMRPGs could be a generator of insights but it proving stuff could get tricksy.

Most of my thoughts are about edge conditions on whether somethign forms or not.

In a way a good sandbox mmprg will have "player driven economy" as one of its game mechanics. In this way having "economic authenticity" has a somewhat solid grounding. however it also has the dark sides in that if you design something in and then measure that thing out then off-course you are not going to get surprised. A lot of what makes a collection of mechanics "an economy" places burdens that it has no hope but to work boringly.

There are quite a lot of things that have been "RPG takeaways". I started to flesh out some of them but decided just to list and let it be interactive whether further elaboration answers the question.

*Ruthlessness from Majora's Mask: Eve lets you play a scammer. If you make a very big slight in game but do not cheat while doing it the house will back you up 100%. When people have proper anonimity and pseudonimity reputation effects can grow rampant. 

*Blowing up pixels: Having a lot of wealth and having hard time to enjoy it. Buy expensive ships just to wreck them for marginal humor utility. "objective" ingame economic value being radically different from subjective evaluation

*You need money to make money: Progression curve and expectation where new player earn little and more established players have it easier. Income differences can be way bigger. making millions in a 15 min vs working 6 months to get to that point.

*Slavery for the sake of entertainment: Time sinkhole, all gains are fictional/experiential, possiblity of getting real hurt (loss of assets worth/generated by 1/2 a year of monotonous repetitive action). People can understand what a grind and ruthless thing it is and still undersign, instead of winching away as a violation of basic human rights.

*Uncommon sense: Security ratings and alpha vs beta. The thresholds of gain vs risk of loss are spychologically very different than what is mathematically good for game balance. One could think robbery possibility vs no-robbery posibility woudl be in the real of 1.0 vs 1.3. When is is more 1.0 vs 3.0 or 1.0 vs 10.0. How people can be financial incentivised to upkeep such sensiblity gradients. Also 3 months in high sec vs 3 weeks in low-sec for comparable gains.

*Adversial economics: Trading on the open market vs company internal market. Making ammunition cheaper for everybody means making it cheaper for your enemies. it can be important political/strategic point of keeping a price artifically ineffcient. The market price might not be the production price but what it can be allowed to be. There is a difference between company internal "market price" and open "market price". Flooding the market as a causus belli.

*Created scarcity: Extortion like tactics to actively make more people more dependent on others

*ISK=RISK: Somebody will do it for less until to the point where it becomes unsustainable because they die too much from doing it.