If you would consider using a fancy wooden knob to fasten your strap to the butt plug, this is what I would do.
You can get small decorative wooden finials at most hobby shops or building supply stores (shouldn't cost more than a dollar ) get one that requires a 1/4" drill to install. Then drill the new butt plug with a 1/4" drill about half way through. (or just a little deeper than the finial requires) Continue this hole through the butt plug using a 1/8" drill. Next get a #8 wood screw about 1" long and put it in this hole . apply some wood glue to the two surfaces to be joined then screw them together. after the glue has set install the finial in the 1/4"hole and this will hide the screw head as well as give you something to attach your horn strap to.
hope this helps;
Soggy