Hi!
A couple of years ago, following the A2000 schematics, I built a board for connecting a CPU slot accelerator and Zorro II expansion boards to the A500. As a Zorro II riser, I used the A4000 one. It gives me some problems when I enable the CPU caches, however what an improvement over stock A500 speed :-)
The idea is of building everything into a custom tower case, but university doesn't leave me much free time... Sooner or later I will complete the project, with the idea of enabling the video slot too (should only be a matter of taking the signals from the A500 board).
You can find some photos at
http://www.gratteri.tk (follow the "Computers!" link at the left, then go to "Amiga 500 Professional"). There also is a text in English which gives a brief description of how I did it.
Ciao!
Daniele