One more! Will think about this one also. Thanks.
But given that this sort of big-bag use on this particular (1999 Joe Starck stripped-down fixie gofast lightweight) will be very sporadic and occasional, and given that I still hope Chauncey won't take 6 months on the new "Riv-Road-like errand bike*" frame, I am thinking that, after all, the solution that may be simplest, most secure, and sufficiently fast might be simply to replace the OEM straps on the Camper with longer ones, or even toe straps, and insert these with both ends on the outside of the flap so that it will be much easier to strap them to and unstrap them from the Flite's saddle rails. But I'm going to try the dowel suggestion above first. I expect that one would need to strap the dowel to the frame rails as well as strap the bag to the seatpost, to keep the dowel sliding forward; but I'll give this one a hands-on try, and report if successful.
*Just for the record, this is a project that Riv would probably not touch with a 50 ft pole, even tho' it is to be based on my 2003 road custom, because it is designed around a hub gear with all sorts of related and complementary, and all very weird, custom braze-ones and internal routing features, not to mention custom racks. So I'm not taking bread or money from Riv's mouths or hands.