Try asking Microsoft to support RT in a halfway reasonable manner.
Really, there was no good reason to require Windows RT to only run special-sauce Metro programs. linux has a strong presence on ARM devices, and most linux programs compile all right for ARM.
calibre certainly does, with no changes whatsoever.
But Microsoft has locked down what you can do inside the Metro runtime, and for all intents and purposes I think it would qualify as supporting a new OS target.
It also can't even be considered until if and when python ever works on Metro -- but that is only planned (ish) for python3 and calibre uses python2.