OK, not finding a good commercial solution, I built my own.
At first, I wanted to find some kind of existing hard case and just build a bracket to attach it to the bike. This didn't work out well as cases were too large, or not supported well enough to really take the type of mount I was looking at building.
So, I built a heavy steel cage into which I could slide my soft rifle case. This cage holds the case fairly tight and is strong enough to easily protect the rifle from a tip-over or minor crash. In fact, I can stand on the cage and it takes my 220 lbs with just a minor flex on its weakest part.
The brackets are constructed out of 1/2" plate. This is mounted to the solid aluminum tailbox mount and creates a very stable and strong mounting point as shown here:
Onto these 9/16" grade 5 bolts, I attach the cage like this:
and holds the rifle case like this:
Seeing that I'm in MA and they have some pretty strict transport laws regarding "Assault Weapons" and this solution is compliant, I suspect that it would work anywhere.
Performance is great. Big bumps do cause the cage to hit the scooter's body, so I'll need to add a small rubber pad at the contact point to save the paint, but the rack is pretty rigid overall.
While riding the rack puts most of the case into an existing airflow and I don't feel any performance issues even on the highway at 70mph. The top of the bag does flap and I can see it in the mirror. But I don't think it matters. That's the barrel end of the rifle, so there isn't a lot of support for the bag up there.
The Waller and Sons bag has a built in lock which complies with MA law for a locked case. I further use a shotgun style cable lock to secure the case to the cage. I have no illusion that this is 'secure' as anyone with a 9/16" socket set could remove the cage and rack. But I'm also not going to be leaving this rig anyplace with the rifle on board.
In any case, I can now take the 70mpg scoot to rifle shoots instead of the 9mpg 3/4 ton V-10 pickup.
Anyway, that's my solution to the problem.