Wasn't there a guy in here which used an AGP-to-PCI converter successfully with Mediator and Voodoo 3? Perhaps you could try the same thing with Voodoo 5 and see if it works with Prometheus?
Even Elbox confirmed it, that with AGP-to-PCI adapter, Voodoo 3 AGP becomes exactly the same as Voodoo 3 PCI.