The zen of Chumby is not to be trifled with. Now that's established we need to follow
the Dalai Lama in learning the rules in order to intelligently break them,
I suggest a re-examination of the human to Chumby bidirectional concepts.
Ideally the truest Human>Chumby inputs at the local level Chumby are screen based widget buttons or the squeeze sensor. Possibly I am overly restrictive ? Then again here comes the rule breaking while allowing soft default for those not having what you propose adding. Go on and mod software&hardware for extra buttons.
BUT! Write your code so that somehow the unmodded Chumby can "know" it's unmodded and display screen widget buttons in lieu of the mods you propose.
That way- anyone wishing to add buttons can do so-and the unmodded Chumby users still can take advantage of whatever your modded path content offers.
Having stated my take on Chumby Zen and intelligent breaking of it with respect,
I offer a possible hardware hack of plug only complexity. A cheap usb hub and one of the many "dongle" interfaced keyboards fits with your concept. The fewer wires in places where Chumby lives are part of it's zen also. If it seems arrogant of me that anyone but Bunny should be warping Chumby's zen think instead of it as being consistent with the hackable by design genetics of Chumby...