I'm only saying that is better to search for a lower goal in order to take impulse for connecting more complex controlers.
I did not understand this one ...
Your project is a great one if you find the way to replicate the keyboard contacts on the expansion port.
As far as I know, unfortunately there is no way to manipulate the "keyboard matrix" from the expansion port.
If there would be a way form the expansion port, that would be great, and I think my keyboard thing would work on that way, already ...
That would be a whole different perspective ...
Then the project will be easily copied by other users. The trick to share those complex things is to make them easy for others. Then the information spreads itself.
I could share the codes and I could point at an appropriate MCU "board" (just found chineese replicas of the arduino micro on e-bay at about 10$ + free shipping, I am courious, will those work or not),
but these are not enough ...
One has to dissassemble the enterprise, soldering many wires, and upload the software to the MCU before that.
So it is not an easy way.
If the expansion thing would work at least, that would be a different case ...