So good news and bad news re: tapping 12v directly to drive the fan(s). The good: it works fine, as expected. The bad: the switch apparently looks for actual power draw on the fan header to determine fan health. Even if the tach line is hooked up the switch reports that the fan is failed if there's no power draw. I think I knew this already but I was still hoping I'd get some joy by hooking up the tach wire, oh well.
I used 3M snap-on quick-disconnect T taps for the 12V feed and cannibalized an old molex fan adapter that split the tach wire off so it's easy to disconnect everything and plug the fan back into the onboard header. I regret tapping the internal supply directly instead of the distribution board but I don't have a contactless multimeter to figure out voltages on the output wires. I'd tap in there so the mod supports both internal and external PSUs if I were doing this again.
FYI: the Sunon 12v 40x40x20mm maglev V3 fans are perfect for this mod, they're cheap, almost silent and move a decent amount of air -- I definitely recommend them. I wouldn't use them without running directly off 12V though, they draw too little power at low speed for the switch to recognize that they're still alive and you'll get "fan failed; fan on; fan failed; fan on" notification/log spam.
(Fan notes: I know that the Sunon VX model will work as a drop in replacement without issue without the 12v mod, I'm 99.9% certain that the V1 will too, I'm unsure if the V2 draws enough power at low speed to be detected as working and as above the V3 will work but spam your logs with fan failed notifications. One of these days I'll buy a cheap scope and cobble together a filter for the PWM circuit and try all of them again for confirmation.)
The whole thing took about 10 minutes, the most involved part was modifying the power/tach splitter power lines with some spare 16ga wire so I could crimp on the quick disconnect spades, the fan wire on my DI(wh)Y splitter is way too small gauge to get a good crimp on its own.
Edit: Pictures:
https://photos.app.goo.gl/AC2iMCVJqNdJu9aT6