Green Flash,
IIRC, It has been looked at a couple of times. Got rejected both times. The first time the argument was that it was supposed to pay for itself over the remaining life of the fleet, but no one would fund the money up front. The USAF/DoD also, as this year, wanted to reduce the size of the fleet so were unwilling to accept a program based on maintaining the size of the fleet. They also made the case based on higher reliability and savings on maintenance, but there were arguments about maintenance intervals and costings.
The second time round the additional case was made that the extra endurance/range would relieve the load on the KC-135 force. But, to be honest, the hours they actually fly a year I wouldnīt it would have made a lot of difference.
Itīs a nice idea, but there are so many higher priority calls on any available funds, its never going to happen.