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How stupid would it be to drill holes for the bells screws to go into (instead of just pushing against the metal)? I imagine it would help make it more secure.

Not stupid, but you have to make sure they’re aligned and the screws are through

Make sure that the bell is in the perfect position before considering this.

Would the metal on metal contact be different enough, so as to make the screws "buzz" when vibrated?

I've done it on several horns. Grind a point on each screw. Tighten the screws a bit to leave a mark on the bell tenon. Then use a punch to make the marks deeper so the drill doesn't 'skid'. Then drill the holes in the bell tenon through about 5/32" diameter or maybe just a dab smaller. That way the screws don't actually go through the holes. That will eliminate any possibility of creating a 'buzz'.

An alternative is to turn the tip of the screws down to about 1/8"diameter for about 1/8" leaving a flat on the outer diameter of the screw. Drill 5/32" diameter holes in the tenon. The end of the screw will pass through the holes in the bell tenon and then bottom out against the tenon. Again... pressure will be against the tenon to eliminate a possible 'buzz'.

In both cases you can avoid alignment issues my grinding a small line that crosses from the tenon to the receiver.

If you look at this bell you will see that ovals have been drilled in the tenon for the screws:

Thanks for the help everyone! The real issue is one of my horns (a Grand Rapids York BBb 3 valve top action, front bell. Not pretty) is that it is damaged and is a bit out of round- bell doesn't stay on perfectly. If I make a move, I'll update the thread.