Thanks for the reply!
I managed to find a 1.6.25 version (which was the last of the 1.x tree) ported to the Atari ST, which reputedly works on the TT, too (not sure, I haven't tried.)
Digging a bit deeper into the code, MINIX uses the segmentation characteristics of the 8088 processor as a sort of proto-MMU, which the 68000 clearly doesn't have with its linear address space.
BUT I was thinking of replacing the STe's 68000 with a 68010, which implements virtual memory and emulation. So it should in theory be possible to emulate an MMU in software with the 68010 hardware support for emulation. That should provide a much more efficient way to handle memory than the limited 64k segments of the 8088, and the current "shadowing" software hack on the 68000.
The Mac port is really sad; it runs as a program on top of Mac OS, so it's not really a true OS. I suppose they couldn't do much because there is really scarce information about internals of Macintosh systems. I have an SE/30, and one of my projects was to port MINIX properly to it, but I am still looking around for information