So to answer my own question, in case it is of the slightest use to anyone else ...
If you install a graphics card with a GPU like an N-Vida GeForce, PE3 magically has GPU Efects and GPU Transitions included in the plugin list.
One of the transitions is "Sphere" which wraps the video around a sphere which then floats off and up out of frame.
I stuck the clip I wanted to have this effect on a track on its own, and added this transition to the end... Then dragged the transition to cover the entire clip., and voila!