Currently the Screenshot request is served by a custom dialog, which doesn't have region mode. Since Spectacle is now part of Plasma, how about launching Spectacle instead, so we have less duplicated code, and full feature of Spectacle?
Interesting idea. We could also go in the other direction and make Spectacle use the portal. This would also allow us to get it onto Flathub more smoothly. TBH I kind of prefer that idea.