Edit: beat by Akien to it, but here is my similar reply for what additional insight it might give:
Ah, the Megaglest forums are accessible from the Philippines again? I was kind of left out the last couple of months since I temporarily moved here for work reasons
IANAL, but I think both sides of this age old argument (repeated very similar in every "is the GPL extending to the artwork of a game" argument) have been mentioned in the thread you linked. In the GPL case it is the "mere aggregation" clause, while for the CC-By-SA (-NC) is it the question when a combination of works forms a new work.
Generally speaking the CC-by-SA is usually considered to have only relatively weak copyleft, more in line with the LGPL, meaning that only very strong integrations are effected. So for example if you render a movie with CC-by-SA assets, the movie will have to be CC-by-SA as a whole and you can not mix it with the -NC equivalent. But if the CC-by-SA files are still easy to extract from the combined work in an unchanged version (as is usually the case of games), than my educated guess (and that seems to be the majority consensus) would be that the CC-by-SA does not go into effect for the full game and CC-by-SA-NC files can indeed be mixed.
Of course you can still not sell the game unless you remove these files, which in most cases will render the game non-functional...
However, these -NC licenses are a very slippery slope, that I can practically guarantee you will lead to license headaches sometimes in the future. So I would advise you as the project lead to simply not allow -NC licensed files for integration into Megaglest for practical reasons (and maybe FOSS ideological, including that -NC is not allowed in most Linux repositories).
Edit2:
if you modify a blender model, the modified version should be set under a SA-compatible license and you should share its sources.
Sadly this is half-incorrect. The -SA option (contrary to the GPL) does not have a source requirement. For some this is a blessing, as for music etc. this is often difficult to provide, but for image files and 3D models it would be really great to have the sources