At David Boddie's suggestion, have dedicated part of this weekend to explore how well PyQt can be used to write Sugar activities. The results have been very satisfying, I would say thanks to python's flexibility and to the Sugar activity architecture, an excellent work of Marco Pesenti Gritti.
So I got something that looks and behaves similarly to activities compliant to the Sugar Human Interface Guidelines, with the following exceptions:
* No combobox for selecting the sharing mode.
* Some theme quirks were fixed with a custom stylesheet, I think these should be treated as bugs in either QGtkStyle or the Sugar Gtk theme. Or if QGtkStyle is not up to this job, we would need to implement a new QStyle. Couldn't workaround all of the issues in the stylesheet.
* No palettes at all. Hope implementing them in Qt will be much less work than it has been in Gtk+.
* The XCursor theme is not picked up, need to investigate further.
* Some Gtk stuff is still used, I think PyQt activities shouldn't use Gtk+ at all for better memory usage and startup performance.
Good news is that I have reimplemented good part of the activity API in PyQt quite quickly, but we may want to design a better API learning from the mistakes of the past.
Now, let's let the Sugar platform get flooded of KDE Edu activities :)