You can also try using an empty QWidget:
w=QWidget()
and using that as the parent parameter in getOpenFileName()
I do something similar in my KindleImport plugin (which also only uses the FileOpen widget):
https://github.com/dougmassay/kindle...ilities.py#L45
It uses the wrapped QApplication from plugin_utils.py, but that should not be strictly necessary to get things to work.