Here is something I came up with. This is something that would be permanetly attached, but is simple and small enough to not be in the way.
This drawing was done with my laptop, so it looks like elephant dung this just gives an idea
The Red is a luggage rack,
Yellow is a AR15
Blue is the actual place the gun would mount, a piece of angle Iron that is welded close on the end, mounted to the swing arm, or can mount in the place of the passanger pegs. Then another piece of angle iron mounted to the luggage rack. The bottom piece that is welded close will keep the rifle from dropping down, then the angle iron acts like a channel, for the rifle to rest on.
Purple is some sort of soft strap. The purple is by the blue, I didnt realize the colors are similar
This is just one idea, this would be very easy to do, and you wouldn't have to install and remove a bracket whenever you transport a rifle.