Don't know of any aftermarket plugs,...least haven't run across any. Sorry.
Are you using 777 synthetic? If so,how much?
I shoot 777 (80 gr) in my Huntsman and find that it will start to seize the plug after about 8-10 shots. I take my supplied tool and crack loose the B-plug at the range and finger tighten while the bore is still warm. That helps.
I also have tried BoreButter on the threads,to experiment,as you have. It worked,but after about 12-15 shots,barrel went to the "vise". :x
Best I have found was Knight Breech Plug Grease. I also found that if I put some grease above the threads and up to the end of the plug,it helped greatly and pretty much ended my welded plug phenomenon. :grin:
Using P-Dex, APP and Goex black powder hasn't reacted the same "for me",but does not guarentee that others haven't experienced a seized plug problem using the above.
Best that I can advise is to make sure your plug and threads are clean before applying liberal anti seize and apply also above threads. Follow installing with Finger Tight Only and loosen periodically at the range to insure the dreaded WELD does not get a chance to turn into peanut brittle.
Good luck