Even the Flight refuelling trials for Tiger Force wouldn'y have been needed anyway, as the invasion of Okinawa in spring of 45 gave the US access to enough airfields, more than they needed, and the decision had been made to allow RAF/Commonwealth AF's use of the excess capacity available on Okinawa for Tiger Force.
Kadena to Tokyo was within the normal range of a Lanc anyway, so even if the Silverplate Superfort's hadn't been available, a Lanc could have done the A-bomb drop from Okinawa, although, for security they may have still wanted to use Tinian anyway, which was beyond Lanc normal range so would have needed that Flight refuelling kit. I would have though the Lancs lack of service ceiling compared to the Superfort would have been more of an issue in terms of dropping 'the bomb'.....but I guess even a high risk one way trip option might have been considered at the time?