Status update:
I got it running on 5.1.0. Quite a few things changed under the hood, so this will still require a bit of work (a lot, actually). So don't hold your breath.
In the mean time, I made the
source code available. Beware: it's rather messy, undocumented, and could need some refactoring. The version that is currently checked in should, in principle, work on both 5.0.* and 5.1.0. But it's totally ugly on 5.1.0.
FontHack 1.3.0 will most probably drop support for KT firmwares < 5.1.0.