Some 25+ years ago I was the FAE/Sales engineer for the local distributor of some brands of VME boards here, so I had some training in the VME bus at Force in Munich for instance but I can't claim I am an expert anymore but to some degree I know the bus quite well. Not the MESS bus/slot system however.

VME is basically a multi CPU/Master capable bus with Interrupt/DMA support and priority levels etc and there can only be one Bus Controller (which can also can be master).

The special case here, I believe, is that a VME CPU board can be a Bus Controller in its first instance but not in the second and beyond. Is it possible for a CPU board to instantiate itself? Otherwise we can just derive a base board to be the controller instance and another to be the slot device, let me know what you think.

Some random characteristics of VME boards from top of my head:
The controller provides the Bus clock, the arbiter and also monitors bus collisions and such. The Bus master is the board that currently got the bus from the Bus controller. The slave is getting tasks assigned and will usually generate an interrupt when it is done or it needs to be polled. A bus interrupt can target zero or more boards, usually one.

I have not coded so much yet of the VME driver so I think I just open a branch on my fork and work there until we got something that you are comfortable with, I'll post it here when I have something.

Because I can