Well, in the end I just bought a second handed A2000 Mobo for like 5 euro's.... On to the next project :-) Here we have a Sonnet Crescendo 7200 model 500 MHz G3. It is fabled that this is the infamous SharkPPC from Elbox. Anyway, here it is installed in an A3000 with a Mediator board.
Took me a while to find the 5V EDO or FPM memory for it, but Ebay (and patience) is your friend. It has now 3x16MB installed on it. Doing a pciinfo (a tool from the mediator package) just hangs the A3000. We'll look at the problem in the pci.library next time.