It would probably be easier to port Safestrap, since it works on the AT&T and Verizon Note 3s, and they're locked down about as tight as a phone can be and still be used.
How easy? Give Hashcode (the dev) a couple of F7s and he could probably do it pterry easily. For anyone else? If you're familiar with TWRP (from a developer's point of view, not user's point of view) it shouldn't be too difficult to modify it to run on any phone with enough storage (assuming there isn't already a version of TWRP for the F7).
Making more than one slot available at a time seems to be tricky, though, since Hashcode hadn't gotten it to work on 4.4.2 the last time I looked. But you can still have a few ROMs backed up on the SD card and restore whichever one you want to run. But, after running a few ROMs on each of a few phones, I find that I tend to end up preferring one ROM, and stick with that one for a long time.