Say hello to the Hale UMC32 USB MIDI Microcontroller, a nice little circuit board brain that allows you to hook up anything you want as inputs and will do the majority of the legwork in converting their signals to MIDI output via USB. Providing you can solder a reasonable front end circuit (i.e. you can make something that wont blow the fuck up when you plug it in), then you can start building your own custom MIDI controllers and interfaces.
At £90, its pretty costly, but the possibilities are almost endless. The DIP switch allows configuration of digital/analog ins and outs, allowing you up to 32 channels of MIDI over USB communication or 64 with two linked together.
I’ve got some big plans for this beauty when i get some cash to play with!