I have a bike with the Nexus 3/coaster brake. It has not been motorized yet, but I do plan on powering the left side. I am planning on having Jim/Manic Mechanic make me a Sprocket Adapter.
If you choose to use a jackshaft setup and power the right side. I have heard that the nexus planetary gearset is a little weak, and does not do well with being powered.