You can bolt one in / under / on the side, and map controls for it. The MVP is duct tape but it's possible to get a small synth & battery into the case.
I know it's not quite the same thing as you're asking for, but when you dig into the linnstrument it turns out to be very hackable.
I know it's not quite the same thing as you're asking for, but when you dig into the linnstrument it turns out to be very hackable.