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I've been involved with mods and other shall we say IP-dubious media. You should be safe so long as there's not monetary benefit from it. If there is, however, expect a cease and desist at some point. They *usually* don't ask for damages if you're cooperative, from what I've seen.

That should make it a lot easier, then. afaik, they're not particularly litigious, unlike say, Nintendo. A denied permission/Cease and desist are still likely. Personally, I'd work on it a bit, make sure it's fairly polished and is genuinely cool, then ask permission.