Ok, so I guess I could just check for Animations in NodeContent.Animations and ignore whether a node is BoneContent or not. Hopefully I could then map those nodes back to the original nodes on the Mesh skeleton, but I very much doubt about that.
What we are looking at is a rewrite of AnimationsProcessor, MeshHelper.FindSkeleton and MeshHelper.FlattenSkeleton. This way could just skip the diferences between assimp and XNA. The importer completely ignores NodeAnimation.
I don't expect to start working on it any time soon.