I would think you only need to create the triangle selcetor for the model once and then use it to initialize a collision animator for the models IAnimatedMeshSceneNode. That should take care of the collision for that node every frame. Hope this helps.
This is a very good question. I think that the triangle selector holds the triangles only but the collision response animator keeps trace of how the triangles are moved rotated or otherwise modified as in an animation.
I know this is the case for objects which are moved and rotated, as a while back I did a school project in which I moved and rotated an object and an object collided with the moved and rotated trinagles in various positions. Whether or not this is true for animated meshes I am not sure but I would imagine so. Just do a test and create an animated box that moves far from its position and try to collided something with it at various points in the animation.