I am right on the edge of trying to build a TinyG based system, and have also been pondering options to set it up without a computer dedicated to it, and some simple UI option. I had to go look up the Chumby One, as I’m not in the small intersection.
That said, I was thinking of an Arduino with an SD card shield, to load g-code, and an even simpler UI via and an LCD keypad arrangement. I was also assuming jog/homing, status line, and file selection, all accessed via an ATM-like UI.
I understand using the Chumby because you have it, but was there some other reason you dismissed any sort of Arduino-based option?