Re: Midi Expansion For The Plus/4! Well, it's not yet public as I'd like to test it first (I literally designed it the other day), but if you want I can send the project to you or I can add you to the still-private GitHub project if you have an account.
I'm working on improving the project though. I have added MIDI-over-USB capability since I didn't have a keyboard handy and I wanted to see how well it works . Not bad for a couple of hours' work, given that until this morning I didn't even know anything about the MIDI protocol!
The board can be redesigned with an Arduino Micro in place of the Nano, which costs just a couple of Euros more and would make this possible.