I agree with others that the whole static fear about using metal pins is of no concern. Brass tacks conduct electricity even better than steel and they would not be of any concern either....at least to me. A horn is a lousy pressure vessel and will not add much potential danger to a horn full of powder going off. Even without the horn, if you put a half a pound of powder in a paper sack and set it off in your coat pocket, you will have some trouble. I think a well sealed horn is the safest way to carry around a low explosive like black powder, wood pins, steel pins, brass pins....whatever.
On the subject of priming flasks, that is a slightly different animal. A while back, Dennis reported on a friend of his who severely damaged his hand when a priming flask blew up on him. At the time, I was working on a primer that had a "blow out" safety feature much like Taylor's suggestion. It uses a modified plunger tube on the business end, but the rear end of the reservoir tube is not closed with metal. The end is plugged with a leather disk sealed and water proofed with a few drops of varnish. It only takes a few psi to blow out the leather disk, so, if this one were to catch a spark, you might get burned, but the tube will not explode and there won't be any shrapnel. (You could do the same thing with a little cork in a plain tube.)
Perhaps a little overkill, but then I had some time, some brass, an idea....and I like to keep all my digits in place.