Pan, dont take it personal, no harm meant. I said "most full..quilts" i did not mention JackrBetter products specifically. Im telling the OP what I hear from the field, people i know personally with mucho experience using full quilts with the blackbird. There has been recent discussion of this issue on HF as you must know.
2 of my buddies both complain that the footbox extends beyond the usable range of a full UQ and that they had cold feet.
OP: a 2/3 UQ will solve two major issues.. bulk/weight and gaps under lower legs. This may or may not be a problem for you. each hanger has differing needs. I suggest you at least consider the idea that a 2/3 quilt combined with a (sit pad, pack frame, etc..) ccf pad such as the GG Nightlite will be a great solution.
after all, this isnt rocket surgery or brain science so im sure you'll get what you want. peace, mike
p.s. as you must know, Brandon @ Warbonnet outdoors designs the 2/3 length "Yeti" underquilt specifically for the Blackbird. He does not do full UQ's. Im not sure exactly why, but i suspect they work better together.