Well that suggests the BPPC port is little more than a header on the BPPC's bus... If it were a PCI port the G-Rex would require little logic to bring that out to standard PCI slots 
I remember asking Ralf Schmit about the port some 13 years ago, and recall him saying it was standard PCI... But I don't know how much he know about the hardware... He could have been referring the whole interface with the BVision...
In the old VGR site there were interesting pictures of the prototype used to create CGX Permedia2 drivers and it had a PCI slot with a standard PCI card plugged in. I guess it didn't have much glue logic. There were other pics like a nVidia TNT(2?) driven WB on a G-Rex.