I've done the seat pin mod on my 2006 1300c. Can I still use a mustang seat?
Not sure if this would solve your problem or not, but I replaced the acorn nut with a wing nut. Of course I had to take my angle grinder and shave the wings down a bit to keep it from hitting the seat as I screwed it down. Not as pretty, but it does the job.Work good with my Ultimate. Don't see why it would not work with the Mustang.
Now I'am trying to come up with something for a quick disconnect for the rear passenger seat. Pain in the butt to get at the nut, especially under the trunk .