Well, I held this post back on purpose. It's never fun to admit defeat.
So far my tests result in a non-booting A2000 with flickering purple video output.
Really discouraging! I will do some more fault-finding, but there's really nothing more than PCB tracks in this design.
What could possibly go wrong? (famous last words...)
I'll keep you posted, but don't build this adapter for the time being!