I've only ever seen one case of a man who'd previously had a rationalist mate going back to nonrationalist mates afterward.

I have dated rationalists and gone back. Rationalist subculture affiliations count very little to me. It doesn't make people all that rational and does make people more annoying when they are, in fact, being irrational. I do enjoy having some shared interests with those I date but honestly I'd assign more 'attraction' points for a fitness obsession, enjoyment of games (board games, cards) or, say, medical knowledge than "being a rationalist".

The reason why the gender skew of our culture is a mating problem for men is that once you go rationalist you don't go back.

That sounds like an argument that one shouldn't date a rationalist even when an attractive option is willing and available. You don't want to permanently degrade your future options for (possibly) short term pleasure with what is immediately before you.

"Go to the physics department, find a woman you consider attractive, point her at HPMOR, and see if anything develops" sounds like more useful advice to me.

If you say so yourself!

I don't know, if a woman had tried that with me she'd have found I didn't make it through to the end (didn't read the last batch after the pause before it). And she'd find that I argue with the author, rejecting some of the "rationalist" morals he promotes in the chapters that get preachy. If she is too enarmored of the work it could disqualify me!

For (straight) men who insist on dating externally, asking a woman whether she would prefer a certainty of $500 or a 15% chance at $1 million seems likely to be a surprisingly good filter on potential mates.

If I happen to marry (or otherwise have significant resource sharing with) a woman who is poor at this kind of decision making I'll first make sure she is willing to let me have final say on critical financial decisions. (Irrational and stubborn or egotistic about it is what would black-ball her.)