Shuttle Buddy has been offering a gas powered friction drive kits designed to fit in a backpack and get downhill riders back up the hill. They are planning an electric version to green up the product line.
However, none of the existing systems will work with a full suspension mountain bike like the Shuttle Buddy system. It has a pivot that allows it to move with the rear wheel.
The Shuttle Buddy system is designed as a single unit that slides onto your seat post. Proper tension is provided by a spring that attaches near your rear wheel skewer. The whole system is easy to remove can fit in a back pack.
The gas system only weighs 7 lbs but no doubt the electric system will be heavier. Stay tuned for more information as we get it.