I just went through the code in kpaint that is invoked when saving the image. 
Basically it all comes down to QPixmap::save. Unfortunately I can't find a 
single place where a QWidget* or sth. similar is passed that could be stored 
for a start. So I doubt it will be possible to find a BC "plug-and-play" 
solution.
We could still play tricks with QImageIO::setPreferences. That would require 
the application that wants to use the new functionality to not use 
QPixmap::save, but implement the code from that function (4 lines if some 
checks are left out) in-line. Then the QImageIO is available for 
manipulation.

Michael