the mode switch for the Commodore 64 models. It's always a bit strange, but I need to make the keys on the plus/4 keyboard send appropriate key presses to the PC so that Vice will reverse the mapping and show the key I actually pressed. This is based on position, so you press ctrl on the plus/4. It tell the PC you pressed tab. Vice says, ah, the user pressed tab, so I'll interpret that as ctrl. Strange, but it works well and give you the surreal experience of using a plus/4 keyboard to drive a plus/4 emulator.
plus/4 USB keyboards are now for sale on my Etsy store. I haven't listed a Raspberry Pi version, but there shouldn't be a problem fitting one in.
I now have a Commodore Plus/4 USB keyboard kit if you wish to convert your own.