There is absolutely no issue licensing game data under the (L/A)GPL. In fact, this is required for at least the GPLv3 in that the license applies to the "whole of the work, and all it's parts, regardless of how they are packaged". Thus if the game code or any dependencies (ie, the engine) are licensed under the GPL, the data must be licensed under a GPL compatible license (which the CC licenses are not).
After numerous conversations with copyright lawyers on the specific subject of games, the entire game is one copyrighted work.
