...are you talking about those rubber bushings that allow the bags to flop all around?
I made two small modifications to my ST 1100 to reduce the amount of saddlebag flop, but still allow the saddlebags to move freely
if necessary.
The first thing I did was to drill two holes in the rear mudguard, exactly centered on where the large rubber bumpers of the saddlebags touch the mudguard. I then installed two large bolts in each hole, choosing the bolt head size so that it was almost exactly the size of the hole in the rubber bumper on the saddlebag. Now, when I install the saddlebags, the rubber bumpers fit over the bolt head, and that minimizes any fore/aft or up/down movement of the saddlebag once it is fully installed.
Bolt Head Size relative to hole in Rubber Saddlebag Bumper
Bolt Installed in Rear Mudguard
Saddlebag Bumper fits over the Bolt Head
After completing the above modification, fore/aft and up/down movement was minimized, but lateral (outward) movement was still unrestricted. I reduced unwanted lateral movement without totally preventing movement by fitting a 'Bungee Buddy' eyelet to each saddlebag, then running a bungee cord behind the licence plate, hooking the bungee cord onto the Bungee Buddy attached to each saddlebag. This substantially reduces unwanted lateral movement, but still allows the bags to move outwards if a force is exerted on them.
Bungee Buddies attached to Saddlebags, Bungee Cord behind Licence Plate
I keep my ST 1100 in Europe, and have spent many hours riding at 200 km/h (125 MPH) between Zurich and Dresden. I've never encountered any stability problems on the Autobahn, other than that the front end gets kind of light above 220 km/h (135 MPH), but I don't think that has anything to do with saddlebags.
Michael