Radar,
I have Taurus 2.6 that has the exact same problem. My muffler actually came off in flight but was held in by the cowl. I think high temp lock tight would do the trick however, I am not sure you could ever break the bolts loose again. That's why I haven't tried it. I tried the medium strength and it held for 2 flights but when I set it downl on the second flight it was coming loose again.

I like your approach and am surprised it didn't hold. Let us know what you find that works.