I'm working on a driver for the Motorola SYS1121, a combination of an MVME120, MVME050 SYSCON, and a disk controller. I can get the MVME120, as a slot device, booting to the 120bug console with a little poking in the debugger (it doesn't fully pass one of the exception tests during a cold boot, but I can bypass it.)
I'd like to try to hook up the bus logic and get it talking to another card in the VME address space but I'm not sure how well the VMEbus emulation in MAME works, or if it works at all. It doesn't look like it's hooked up in any of the other VME slot devices.
The firmware and manuals are all on Bitsavers and m88k. Unlike the later MVME SBCs, the 120’s bus logic is all in PALs and TTL.