Okay, this might be a radical stance (and I'm not very involved here, so I can see I might be pushing it a bit...) - but why not just make it mandatory to licence as CC-by-sa ? I have the impression that with every Open-source game, you have the tendency that at the fringes, the open-source idea always has a tendency to be undermined... (there's similiar with Spring)
I mean, my understanding is we wouldn't be playing this game in it's current form, if it's content weren't licenced CC-by-sa. Or am I mistaken? So if people want to contribute to the game (and that's what I'd view submitting maps to the official download server as, after all maps are a central part of the game), then to me it would be quite natural that it's licenced the same as the rest of the content.
Else down the road, you have the ever occurring problems of "oh, this much played and loved map could do with a revamp/new textures/needs to be fixed for the new gameplay logic. But unfortunately the author is not contactable, and the licence is ND (no derivatives)..."
Please excuse me if I'm being offensive to people by saying this, but I just think it's important to view the broader context.