The mesh colliders don't update with sprites, and for good reason - they cost a LOT to create and modify at runtime. You could probably get away with it if you only ever had one sprite you were doing this on, etc.
One alternative way to doing this is to create all your sprites, and then only display the frame you need. Again, this only works if you have a finite set of frames you want to switch between - 2D Toolkit has no such constraint, and implementing something like that into the core system would be a bad idea.
We'll be adding multiple box collider support in a future version (after 2.0) - that way you'll be able to animate complex colliders out of boxes. Its a much more efficient way to do things than using polygon colliders.