Thanks, Nick.
I wasn't clear; I wanted a system whereby the support with bag attached could be quickly removed from the bike, leaving bare seatpost and saddle.
My idea for this particular support was to have removed the "prongs" altogether and simply strap the support, by top rail this time, to the loops (with bag strapped to same rail but inside the outer too), attach that 12 cm-long stand off permanently to the bottom rail to keep the bottom of the support level, and use a third strap to attach the other end to the seatpost. Removal of the whole schmeear would thus involve unbuckling 3 straps.
"Was to have removed ..." I don't know what complex tense that is, but it well describes the situation, because yesterday I gave the support to my brother.
It really is a rather elegant saddlebag support solution for the right saddles -- with bag already strapped to the support, stick in the prongs and cinch the Fastek strap to the seatpost. And, for saddles not right, I think my hack would have worked well. But brother has saddles more apt for this device, and I'd just as soon let him bother with it. Besides, perhaps he'll feel generous and give me that brass Sturmey Archer top-tube-mount quadrant shifter.
For the record, and for the information of those who may have scant room for a saddlebag between saddle and tire or fender: This support would be just right. Even strapping the device by top rail -- the prongs are 2 rails down -- to VO loops would keep the support about 1" above my rear fender, and that in a distance that is very small indeed. Only the SQR system raises the saddlebag higher, and that is clunky by comparison.
This photo shows the scant space between saddle rails and top of fender. Even a Junior will drape the fender when strapped directly to saddle loops. The SQR system does work in this space, but I dislike the clunky seatpost clamp. I will go back to a huuuuge Ortlieb bikepacking-type of saddle bag which trades vertical dimension for huge rearward extension. That one holds a bushel.