Thanks, I've added a bit more info about the connections. I don't have an Amiga 4000 keyboard, but I think they're essentially the same even though they have a different connector. I.e. you'll just have to figure out which pin is which.
As for the Teensy, you could probably use any model with native USB support, but the cheap Teensy 2.0 remains the most economical option. Anything else would be overkill really.
I'll add a bit more to the keymap handling in the next few days, including dynamic switching since I have both US and UK keyboards and would like to be able to switch between the two using a specific key press combo. That being said, adding new keymaps is trivial for someone looking at the code, it's essentially an array look up table ordered as per the Amiga keymap codes paired to their Teensy USB counterparts.
At this time, I'm not planning on this being more than just a nice weekend project to give back to the community. I'll be sure to post something if I decide to build a few units.