Hm… nope! I tried flashing another microsd card with the recovery image. Same problem: screen flashes purple and presumably it’s booting, but I don’t see anything.
Out of a weird curiosity I put a microsd card that didn’t have a system image on it and tried booting it and there was no purple flash, no keyboard rave effect possible, so I really do think it’s booting part of the way on the other two microsd cards. Not sure what’s happening. This literally happened after a dist-upgrade so I’m not sure why that would mess things up for a fresh image too.