"I agree that electric motors could be used for regenerative braking, but the energy transferred from kinetic to electric must go somewhere."
Yeah, it can be transferred in to heat. You put a hefty resistor bank on board and, once the batteries are full, just burn off the speed (literally - almost).
If this idea was adopted...
Firstly, the aircraft would taxi to holding point CLOSE to the end of the runway and then go through the engine warm up procedure. This is what Branson was planning.
Secondly, there would be backup power available at the parking lot surely? Airlines are not that dumb - cheap but not dumb.
Isn't it the main gear that takes the vast majority of the stress on a commercial jet? The nose wheel is mainly there for steering... and stopping the horrible scraping sound of the aircraft frame on tarmac.
And for those bitching that taxiing isn't the problem, well it was a big enough problem for Branson to bother trying to tow his heffalumps. Taxiing is horrendously inefficient; those engines are designed to run fast and high. For an airline manufacturer to propose this solution they obviously think that there is an operational saving to be made (although this IS Airbus...).