[KevinH]

I looked at:

qtbase/src/plugins/platforms/windows/qwindowsintegration.cpp

And that is in the QPA platform plugin code and it is where that command line option is processed:

QWindowIntegration::DetectAltGrModifier but unless we use the QCommandLine processor and somehow in main.cpp before opening the QApplication we try and append "-platform windows:altgr" to the actual command line arguments, it might work but ... I am not sure even then it would be early enough.

Perhaps there is no easy way other than what the Sigil User's guide approach recommends.