Yeah, it'd be a legal quagmire. I'm not sure if copyright can be quite so easily signed away once even implicitly granted and probably varies between jurisdictions anyway. Even if it is possible, just the concept of that sort of thing tends to irritate a lot of modders and puts them off. As for Bethsoft's CC, that's just their latest attempt to monetise modding and that's also gone down like a lead balloon...
And obviously that's my personal opinion, not as someone who has any legal grounding nor speaks in any way for Larian!