Hi, I am just a beginner but would like to contribute to this too as it has been something I wanted in KDE for a long time now(I am a bit sad that this feature request hasn't received much attention). I have had a look at the code and I think the CubeEffect implementation seems like a good example to follow as it handle "continuous" gestures that follow your mouse via void windowInputMouseEvent(QEvent* e). I thought something like this could also be added to the other effects as well (as @cjsoten suggested you could connect a slot to the progress signal from SwipeGesture).
However, I feel like QAction is not the right tool for the job (although I suppose one could hook up an attached property to handle the progress since it is obviously a QObject after all).