I just started looking into this, and found...
https://www.abelectronics.co.uk/p/54/IO-Pi-Plus?CatName=0&c=0You can stack 4 32 bit I/O lines for 128 total. More than enough for every pin on a ZorroII card monitor.
The only question is will they be fast enough?
We would also probably need some kind of ROM to make it autoconfig.
Ideally, since we can emulate a whole Amiga on a Pi, we would just emulate just the peripherals that there are already drivers for. To the Amiga it would be talking to things it already knows. The Pi would do the work of telling the Amiga what it wants to hear.
I wouldn't bother with acceleration. I think we would be taxing the Pi enough trying to make it RAM, mass storage, and networking.
These are fairly generic needs for any system and I don't know how well Amiga OS support multi monitors. We're essentially looking for an RPi Bridgeboard.
