Is there an API for USB and the Kemper? Documented?
No, not anything that I'm aware of. I've analyzed the USB protocol and there seems to be a simple MIDI-over-serial thing going on. It shouldn't be too difficult to write a user space program that communicates directly with the Kemper on all platforms. However, the messages on the bus themselves are not documented in any way and they can change at any time with a profiler, rig manager or driver update.