Ok bare with me here. When I first looked at that patch a thought came to me that maybe it was done from the inside with the butt plug in. Sooo I decided to try an experiment. I took a horn that the tip drilled out to 1/4" put a plug of pine in it, melted a piece of pewter about the size of a good sized marble, hot just like if I would pouring a knife bolster of nose cap, poured down the 1/4" hole and it flowed right down to the plug, it did not burn or scorch the horn, it solidified in now time. So I think Randy was right about using sand to fix the hole only the sand would be on the out side. If the hole was located like the one in the post we would take a pan of semi packed damp, not wet, make an indentations with your finger making a little pit. Take the horn press it gently into the sand with the hole over the pit and the tip up. Melt some pewter pour it in the tip, it runs down out the hole into the pit and some of it stays inside to form a head to lock it in place. It make take a little playing around with but I think it would work but of course only if the hole was on the outside curve of the horn.
Tim C.
PS: If I had a horn with hole in it I would try to repair it like this. Let me say a new horn.
PSS: Ya might want to make sure ALL the powder was out of the horn.