Beags - Not sure about the 787's tanker suitability, but back in 2005, when the 787 was new, I asked why used 767s (of which there were quite a few with plenty of hours left) could not be modified for USAF and others.
The answer was that the -300 had no tail clearance for the boom at rotation because of the longer body and that there were not enough HGW -200s in the fleet.
But then, a few months ago....
