It would be really cool if the other store owners cooperated with GoG on this, like you suggest that Ubi did.
It would make market sense, too. For example, if Valve implemented a “launch-only” mode for Steam which didn’t do updates (unless critically necessary) and just did whatever DRM checks as-fast-as-possible, with zero U.I., it would make me MUCH more likely to buy Steam games and to just plug those into my GoG GG2 unified library.
Sure, they wouldn’t get to waste my time installing a 150 MB update every other day for no reason. I’m sure that would probably be enough reason for them not to play nicely in this instance.
In the customizable integrations file, can you set command-line arguments for the launchers that GG2 integrates with? For example, one could configure it to use Steam only in offline mode. I’d be fine with that since I NEVER use the social features of Steam at all.