Once you got it to boot without secureboot enabled on the hardware, shouldn't it be trivial to do some binary slicing and dicing to cut the signature checking out of the kernel?
It seems too easy. Where does my line of though fail?
In theory, yes. "Trivial" is hardly accurate, but it would be relatively easy. Of course, on hardware without Secure Boot enabled, you wouldn't need to do that in the first place; there are ways to disable the signature checks at boot time (but Secure Boot blocks them).
In practice, good look finding anything that *isn't* a Windows RT tablet but which has firmware Windows RT can run on. ARM isn't much like x86, where everything is designed to be compatible with everything else. Even relatively simple ports of open-source OSes (Linux, including Android, for example) are difficult and typically fraught with driver issues. RT is not open source, and is targeted at very specific hardware. Frankly, I would give poor odds of success. Doesn't mean you shouldn't try, if you get the opportunity, but don't hold your breath or expect somebody to do it for you just because it's "easy".
Good luck! Let us know.
BTW, not sure how useful WP8 drivers will be. Even leaving aside the fact that most tablets won't have much phone hardware (possibly some, though) the kernel is pretty different. They're both technically NT 6.2 (or 6.3) and so drivers may be compatible at the ABI layer, don't be too sure. The phone kernel is very slimmed down and missing a lot of stuff. Win32k.sys, for example, only has about 1/3 the entry points that it does on the desktop (or on RT).