That is legally impossible. Most new features need to be integrated into Qt proper, since they will involve modifications to the existing code. This is why pretty much all of Digia's work is part of the open-source Qt release.
If you are really concerned about a single add-on for Qt being proprietary, keep in mind there are easily dozens, if not hundreds, of open-source add-ons for Qt. Even if you exclude KDE, which is responsible for most of them, and dozens of add-ons released by Qt directly, you still have things like qwt, pyside, pyqt4, and many other independent, open-source add-ons for Qt. So one add-on being closed source is hardly a significant fraction.
Well, that depends on the POV: for me, python extensions are of zero value, while proper charting in the toolkit would come useful often.