Quote Qt5-feedback:

I haven't looked into this issue yet, as it basically expects your application
to use the NSDocument model approach (which Qt applications don't have). So this
would involve much more work. I think basically provide a "fake" NSDocument
structure and map all OS X calls to some Qt signals (from a new QDocument
class?) which would inform the Qt application "now is the time to save into this
file path!" or "Please restore your state from that file path!" etc.

Basically replicating Apple's API, but in a platform-independent API As I
said, I have no idea how complex - but certainly no free lunch...