I'm just getting started looking into Qt/PySide with IDA myself, so I
certainly can not give a definitive answer here, just at guess, but I
would look into something like:
Module idaapi TForm find_tform(caption)
locate a form by its title
e.g. frm = _idaapi.find_tform(title)
Module idaapi :: Class PluginForm
FormToPyQtWidget(form, ctx=<module '__main__' from 'hrdoc.py'>)
Use this method to convert a TForm* to a QWidget to be used by PySide
Doing it with PySide is a little bit beyond me at the moment, but it
should be doable. Qt can not enumerate windows in other apps, but
because a plugin is inside of IDA, so it should be possible to enumerate
all IDA child windows.