@Amigaz
Dragging up this old thread, did you try setting the "swap config" jumper on the mediator card to closed?
According to the manual, this is the setting to use if you are using a Z3 graphics card. Perhaps this configures the mediator for using i/o cards which do not require a voodoo card for DMA buffer?
Unfortunately, I don't seem to have a RTL8029 card around any more (I'm sure I had one somewhere..) so I can't test it myself.