I already thought about exactly the same some months ago while I was building yet another Z80 DIY computer. I think it would be somehow easy to use an SRAM as buffer between the Zorro bus and the raspi. In fact I've already connected an SRAM chip to the GPIO pins of the raspi with transfer rates up to 80 MB/s (peak). This even works with a 5V SRAM without level shifter.
On the other hand, it depends what you call a bridge board. If you just want to use remote desktop or VNC just connect a raspi to the LAN. In contrast a classic bridge board is something that uses ISA (not possible with raspi) and can talk to VGA board, disk controller etc. Or that uses the gfx of the Amiga but indeed this would require a lot of software not just VNC. In fact this was the original idea of a bridge board, using a PC that utilizes as much amiga hardware as possible to reduce costs. Since a raspi has everything, it is basically not a bridge board, just a computer you can connec to with VNC.
However, very interesting project. Question: How to connect something to the Zorro bus? I'm not familiar with AutoConfig and a logic analyzer, Zorro prototype board etc. would be needed.
