This is an older question, and I have no answer, just experience:
When I flew from Frankfurt (FRA) to Geneve (GVA) in January 2015, we were towed by a tug nearly all the taxiway to the runway. We turned to the parallel taxiway "N north" to get rid of the tug, then went back on taxiway "N" to the runway.
Here is our way as tracked by my mobile GPS:
The pilot told us in advance that we will to this, and that it is some kind of experiment / project / study about if it works. (I guess more from logistics / operation side)
From my data, I also have some time information:
12:12: End of push-back
12:15: Leaving terminal
12:21: turn right on "N North"
12:22: get rid of tug, start engines
12:27: Start moving again, and back to "N"
12:29: arriving at runway
If I remember correctly, the engines were started when we were on "N north", so 5-6 minutes before takeoff. However, we were taxiing to that place 20 without engines.
It would be interesting to know how much fuel could be saved by this procedure (It was a A320 or similar, I don't know), but at least, the aircraft didn't need any extra equipment.
(And yes, it was an ordinary tug, not this electrical stuff or similar.)