If the Boeing electric taxi trials are a success, then it's only a short step to regenerative braking. The downside is that you'll need some sort of battery/ultracapacitor which will add mass and decrease the efficiency of the rest of the flight, although if you're not carrying taxi fuel then that is quite a saving...
After a little research, I found a tractor unit capable of pushing/towing a 747. It has a 400hp diesel engine so not that different in power to many lorries. I think the fuel savings at major hubs (LHR, JFK, etc.) would be considerable, given that the tug uses gallons per hour and the aircraft, tons!
Whether after the necessary changes in infrastructure (paved tug return paths), buying the tugs and paying the tug drivers it still proves economic I don't know. I'm pretty certain that there would be a huge saving from not using even a small percentage of the fuel required for taxi at LHR...