Sorry, I did it 2 years ago, barely remember the details I gave it up due to the lack of interest.

Thus,

1. If you want to take care of the whole package and develop it further, I'll aid you whenever you ask. But I'm not planning to develop it further myself.
2. To support qwidget, you'd have to modify the library-helper.py and rebuild. Maybe some changes in a booster would have to be made but I doubt so. Building sometimes stalls, then you just have cancel it and start anew. This doesn't happen in the autobuilder, though.
3. I guess the C++ inheriting shouldn't prohibit it using MDeclarativeCache. But I really don't know.

Please overtake the development if you wish.