The MMU is pretty good described in this document 999-300-1891S_UNIX_PC_Reference_Manual_1986.pdf
on Bitsavers. It is discrete/PAL based circuit so you could write a custom MAME MMU device that interacts with the 68010, place it under src/mame/machine/, at least one custom MMU is there, or keep it in the driver if no other driver needs it. Others might know a better place in the source tree. The firmware package at Bitsavers (http://www.bitsavers.org/pdf/att/3b1/firmware/
) does not include a PAL dump itself as you might know but maybe it is not needed as the description is quite detailed.