MAME's core started out where it couldn't do a bare slot bus where one or more of the cards are bus masters. I don't *think* that's true anymore and you could now create a driver where all it does is host VME slots (or S-100 or whatever), but I haven't tried it either. I do suspect in the degenerate case where you run a bare slot bus with no cards and no CPUs the scheduler would still get angry.
I'd recommend making a SYS1121 driver as Edstrom said that has a VME bus and two of the slots are fixed to start with the MVME120 and MVME050 cards fixed in the slot. (I'm assuming multiple disk controller cards are possible; if there's a specific one, put that in a third slot). The driver itself would be tiny and you'd be following how the real hardware works.