There is a lot lacking in regards of VME specials support but if you look at mame/drivers/miniforce.cpp you get a grasp how a multiboard system is set up as a driver. The boards are implemented as slot devices under bus/vme and can as such be populated in the miniforce.cpp driver or any other VME bus driver. With other words, if you make it a slot device instead of driver you can use miniforce.cpp driver as the VME chassi. But the SYS1121 probably had its own chassi which could be the driver you want to write. Checkout devices/bus/vme/vme_fccpu20.cpp for an example of a VME CPU slot device

Because I can