An idea if you go the steel rod and braze route. Before doing that, file very carefully a sharp "V" along the top of the existing hole deep enough that there will a straight path of pure braze clear across the crown that will serve a a guide for the drill bit. Now, drill that first hole with a small drill. Use that as a pilot.
(Caution - just thought this up. I haven't done it nor am I a framebuilder. I would set up a jig to aligh the file at both ends of the hole. How? Sorry, not there yet,)
Interesting challenge.
Ben