The question basically boils down to whether a game that includes a pile of assets is merely a 'collection' of those assets, or if it is a 'derivative work' made from those assets.

Officially, the Wesnoth team seems to take the stance that Wesnoth is a 'collection'. They allow -ND sprites to be used in the game - if the game were a derivative work of the sprites then obviously that would be a violation of the license.

Personally, what I do is ask the artist and make sure what I'm doing is in line with what they want, because whether or not it's legally okay is very different from whether or not you're gonna piss off people by exploiting what seems - to them - like a nitpicky loophole.

That said, creative commons really dropped the ball on this license, and despite me nagging them on multiple occasions they haven't done much to clear things up - I honestly wish -BY-SA didn't exist in its current form, because it doesn't do a good job of transparently communicating what can and cannot be done (which is the whole point of standard license).