Getting the basics running on Fremantle would most likely not be that hard, since there is still support for C++98 compilers newer than GCC 4 (I don't know if we require any specific version, I think anything 4.0 or newer will do). A more difficult problem might be whether codebases like V8 will work, but probably not an insurmountable problem.

That wouldn't give you platform theming of QWidgets though, because that was rather heavily hacked into Maemo's Qt. It might be possible to resurrect most of it as a style plugin, or perform the same hacks again, though.

[EDIT]: ah, it would also require a relatively new libxcb (and dependencies) built for fremantle - I don't think they exist already, though I don't know for sure.

With regards to the N9 port - it won't give you all that much useful. Fremantle's display stack won't have the hacked up XI2 support that the Harmattan port uses - but fremantle doesn't have multitouch anyway, so that's no major loss.

[EDIT 2]: I definitely don't have the time (or desire) to work on this myself, but I'm happy to act as a point of contact / mentoring from a Qt perspective for anyone wanting to try take this on.