By the way, the reason for the 8086 VDU card freaking out is because of my flawed initial implementation: I had ROM mapped at both F0000 and 30000 so that the boot vector could be fetched.
I've switched to using a read tap which unmaps F0000-FFFFF as soon as the boot vector is fetched, and the VDU card now appears to be functioning more or less normally.
When the first vsync interrupt comes in, the VDU card looks at F8000 (which is located in the half-meg upper window into the Multibus address space). If it's zero, it simply sits in a tight loop, reading the location.
Since I had the boot ROM mapped in that range, it was seeing a non-zero value, then completely freaking out due to what would apparently be invalid data in that range.
So yes, at this point I suspect what's needed to move forward is to get the Z80 up and running.