Would a post about optimizing physical attractiveness be fitting for this forum?

If well written, I don't see a reason why not. Writing well would include stating your level of certainty. From my perspective, there is nothing wrong with "I tried these things, here are the results, no idea whether it generalizes" as long as you admit it. But if you write "this works for everyone" or "this works for most people", expect people asking how do you know what you believe you know.

I wonder (this does not mean your article needs to address this, I am just thinking aloud here) how much "attractiveness" is a coherent concept. I mean, one possible extreme is that there is an objective scale of attractiveness, and if thousand people will rate each other, everyone's scale will be the same. The opposite extreme is that everyone has their own idiosyncratic preferences, and there is no correlation between them; any statement about attractiveness is just a statement of speaker's personal preferences.

Now, I assume the truth is somewhere in between, that different peoples' ideas of "attractiveness" correlate, but not perfectly. Which leads to the question which traits correlate more and which correlate less. (As in "almost everyone agrees that X is better than non-X, but people are divided on whether Y or non-Y is better".)

Is "physical attractiveness" the same as "sexual attractiveness"? Or does it make sense to say things like "this person's body is absolutely amazing, but I definitely wouldn't want to have sex with them" or "this person is completely ugly, but so hot"? Again, the following question would be which traits are usually "nice but not sexy" and "ugly but sexy".

Because many people are sexually attracted to men but not to women, or vice versa, perhaps each trait should be evaluated in two dimensions: how attractive is a man having this trait, and how attractive is a woman having this trait. Some traits (e.g. health) may be attractive in both sexes, some traits may be useful for one sex and irrelevant for the other, some traits may be useful for one sex and harmful for the other; could we perhaps get a 3×3 table of the attractiveness of trait by owner's sex?

How does this relate to heterosexuality vs homosexuality? For example, when male homosexuals judge men, do they use the criteria heterosexual women use to judge men, or the criteria heterosexual men use to judge women? Or something in between? Or does it depend on the trait (like some traits are "perceived by men as attractive" while other traits are "attractive on a male body")?

Is the popular wisdom significantly wrong on some subjects? Like "everyone believes that X is important, but in fact it makes little difference" or "everyone believes X doesn't matter, but when you increase it experimentally, people will be 10 times more attactive (while denying that they are attracted because of X)"?

I apologize if this is too nerdy, asking about theory, when you probably wish to share some practical advice.

But whatever advice you share, even if you support it by data ("I did X, and my dating success increased by 300%, plus all my friends replicated this"), my first question would probably be how much the change was universally attractive, and how it means that some people had a strong fetish for it and those were the ones you succeeded with.

(Also, the placebo effect. Maybe you did X and believed it made you attractive. Actually, X didn't make you attractive, but your self-confidence did.)

It's not a topic I expect to learn from or make much use of, and I think that variations in individual situation and definitions of "attractiveness" are great enough that it makes advice difficult to give or hear without getting tangled up in judgement and perceived judgement.

That said, there are very likely interesting discoveries and experiences you can share, and there's almost no topic that's not appropriate for this forum, if the focus is on experimentation, evidence, and structured approach to important-to-people topics. 

I'd say go for it.  It could help someone, it could get you useful feedback in your endeavors, it could be interesting or trigger side-discussions that are fun, and it's unlikely to do any harm.  Hmm, maybe move this paragraph to the start of my answer.

Lukeprog wrote some related posts a while ago: https://www.lesswrong.com/posts/x8Fp9NMgDWbuMpizA/rationality-lessons-learned-from-irrational-adventures-in [LW · GW]

In particular the stuff on fashion.

I'd like to read it. PM me please if you decide not to post it publicly.