I might have a look at the UAE sources.
If UAE uses the standard Amiga ROMs (not special ones made just for it), then there must be some simulated hardware for the ROM to access. The ROMs would acces the registers of the Amiga's hardware (scsi, ECS, AGA), I would have thought these would be present on a UAE system. I suppose it mught use something custom, but too custom and the ROMs wouldn't work with it, that's my line of thought at the moment.
srg