Hi!
I think that work is already done by mega-hz... on his page is some files that contains photo of PCB with removed parts... there is also z "users manual" in German that describes this memory extension... mega-hz also make a cloned version (probably for ABBUC)... the images of cloned PCB is also present on his website, but it's censored in some way... I think it is possible to make a small reverse engineering and retrieve the schematics. I don't know the legal status of this solution, but the files from mega-hz is publicly available, there is also the photos of ABBUC re-edition of this extension...
The users manual in German language: Compy Shop 256KB Handbuch
Compy Shop 256KB memory upgrade original PCB (top & bottom side):

The photos of cloned version for ABBUC, done by mega-hz:
and megz-hz version of new PCB (as I mentioned above, that is "censored" version:

ps) all files are from mega-hz site. I only converted the "Handbuch" into "djvu" format (smaller size).



