Does the ACA let me just sign up for some really good plan for a few months specifically to cover this event? Are there rules against that?

AFAIK enrollment can only be changed 1 time per year, I know these rules apply to health insurance offered through an employer and I believe they also apply to ACA "plans": https://www.healthcare.gov/glossary/qualifying-life-event/

If the only route forward in the US is so expensive as to be bankrupting, first of all I'm sorry this place is so fucked up obviously, but I would consider medical tourism since it's probably going to be a good bit cheaper, all-in, than just the procedure at a local hospital, especially if there's a hospital stay involved (crazy, huh?).

This isn't legal advice but there are some hospitals that just write this stuff off. Unless you have it on really good authority, I wouldn't risk it, but I have known people to skip out on hospital bills (including me in the past). Anecdotally, TJUH in Philly seems to...

Get it done in Mexico or Colombia

Like I don't know what your thing is obviously but there's a very real chance you can get it done for less than an American deductible including travel and lodging