Basically, from what I've heard, it would be possible to put in a background task (not hard-coded into the OS) that would read the "chip" memory and act on it. Because the 68k emulation is not sandboxed, any older AGA programs would be able to utilise the new features of OS4.
But I'm not much of a programmer, and don't know how (if at all) it'd be done. So no, I dont have "even the slightest idea how much work it would be to adapt UAE this way ". If it would be really hard (and it sounds like it), then just forget it :-)