Wasn't the CUDA ADB controller (341S0788) purported to be a custom 6805 processor? It would be pretty cool if you could pull the code out of one of those. They're found on Macs starting some time around the Quadras and go at least through the PPC Macs.
The supply of scavenged ones must run out eventually. It might be nice to be able to program up a replacement from a 6805 in the proper package.
On the other hand, it appears that sellers on Ali Express have them for sale for $1 - $1.25. Strange. I can't tell if they're new unused stock or used scavenged from old boards stock...
http://www.aliexpress.com/item/341S0788/1884263656.html