Since the question comes up in discussion now and then, figured I'd pass this along.

At the ECCC Marvel Animated panel, Vp of animation, Cort Lane, confirmed that the rights to do an animated X-Men series are owned by Fox.

So yeah, no loopholes there. Fox currently owns all movie and TV rights for the X-Men and a share of the merchandising rights. It really doesn't look like there's much Disney/Marvel can actually do with the franchise outside of the comics and possibly video games.