PyQt has a plugin system for adding python widgets to Qt Designer so that they can be drag'n'dropped into the .ui files for other widgets.
http://pyqt.sourceforge.net/Docs/PyQt4/designer.html#writing-qt-designer-plugins
How could this be done for PySide widgets?
To answer the specific question: "How could this be done for PySide widgets?"
There was once a QtDesigner plugin for PySide that that allowed custom widget plugins to be written in python. However, development on it stopped before it became fully viable. To quote from the pyside mailing list:
On Thursday 24 March 2011 19:18:02 Hugo Parente Lima wrote:
On Wednesday 23 March 2011 22:46:32 Gerald Storer wrote:
I vote for removing the QtDesigner plugin example from pyside-examples, it doesn't work and the support for QtDesign plugins isn't on our roadmap yet, besides IMO it's not a very important feature to have and we have more urgent bugs and features to do at the moment.
Any objections?
Timeout! Plugin examples removed from pyside-examples repository.
So all you have to do is find that old QtDesigner source code, get it working, and then submit the necessary patches :-)