Just thought of this the other day- don't know if it's been done, or really even how to implement it.

Basically- in the tail of the plane, parallel with the wing, install a small reversible ducted fan. Make it work with the rudder servo. Think this could open up a bunch of 3d maneuvers? Or if not, how about in combination with reversing the main motor driving the prop, only at low speeds of course...