I was thinking along the same lines of brad-man -- sew or epoxy in something that better serves the application than d-rings. But I got chewed out because apparently that's un-motobricky and too dealer-noid :dunno
But since the question was posted, I guess it's ok to toss un-motobricky ideas out there anyway.
Since the top of the case on which the bag rests is flat, you could create a solid bottom surface for the bag that will mate with the top surface of the case. One way would be to lay out a couple layers of fiberglass cloth like in the first pic. You wouldn't need more than two layers of cloth to keep it flexible enough to bend to get in the zippered hole, or maybe you'd just want to cut it small enough to get through the hole without bending. You'd maybe want to build up a couple layers where the fittings will attach for strength. Fiberglass is light and strong, so there'd not be too much additional weight. Then you'd just lay the base in the bottom of the bag, drill through the bag to attach some kind of male-female fitting, and water-seal with rubber washers and blue RTV if necessary. A slip hinge would work well there, see pic.
If you point the pin towards the front of the case then you'd just slip it in from the front and the airflow would keep it in place. Velcro flaps sewed or glued to the front of the bag and corresponding velcro strips glued to the front/rear of the case top section would keep it in place. A D-ring on the handle strap at the top would keep the bag's top secured to the side of the bike preventing it from flopping over if the bag isn't full. Or, a single D-ring at the handle attached to the side of the bike might even be enough to prevent forward slip in the fittings and keep the top secured to the side of the bike, you might not even need velcro or anything else. Just detach the top D-ring, slide the bags forward out of the slip hinges and you're done.
Devil's always in the details but the concept is sound.
If the work/cost overhead exceeds what you're willing to expend or it's not motobricky enough or won't work for some reason I didn't see, then fagettaboutit. But at least I gave it some thought, which is what you were asking for when you posted the question.
