That would be really interesting! I was thinking about some kind of adapter (perhaps 3D printed) that could be screwed/inserted into the flashlight head in place of the usual battery tube, and would be able to supply it with battery power (ie, positive to the head spring/stud and negative to the tube) while leaving the flashing pads accessible so we could connect an UPDI adapter to them.
Of course your idea of using an entire separate board is much cleaner and more manageable.
I do remember seeing a post somewhere (here in BLF? on a blog? can’t remember) by someone who did something similar. A quick search brought this up, it’s on the same vein but not what I remember reading: Adventures in TinyAVR 1-Series - #33 by gchart