Actually, seems to work -- I've only tried the "picking" part, not actually creating documents there yet...
As for file manager integration -- I've got Solid Explorer, and it does show when picking *files*, but does not show when doing this new "pick root directory" thing. I guess that part's not implemented yet (I mean in Solid Explorer).
The good part though, is that Google Drive didn't show. It would be weird to have the "save" folder be in the cloud.