From my experience of taxi times at JFK 2,000kg is probably a reasonable figure for a 747 taxiing out at peak times so that could save £1,200 with fuel at $3/gal. I think that Mr Boeing is a little concerned about stresses on his nose wheel though.
Even a 5 minute warm up would still allow savings.
Prior to the introduction of towbarless tugs aircraft were not allowed to be towed during peak hours at JFK. BA engineers would tow a 747 from a remote holding gate (presumably on two engines). I don't know if this still happens.
I've taken off from MIA where the runway is right next to the DL pier less than five minutes after the engines have been switched on, but that was with a JT8D powered aircraft.