I've a friend who owns a metal plating factory and he dissolves studs broken off below the surface of alloy castings using strong nitric acid. That would dissolve the grub screw that retains the rod the gear selectors slide on. Not for people unfamiliar with handling such reagents though!
When assembling these gearboxes it's better to use a short 1/4 Whitworh Allen screw and locknut in place of the grub screw. That allows the rod to be withdrawn with the gearbox in place in the bike. The Goldie racers used that dodge for many years.
Also, carbide drills can be used very successfully to drill into hardened steel like the peg the gearbox camplate rotates on, BUT you have to do it with the thing you are drilling supported in a drill press or milling machine, no need to use lots of RPM, and go easy on the feed. That way you won't break the cutting edges on the drill bit. Carbide drills don't like being used in hand-held drills as the cutting edges shatter.
Hope that helps,
Jon
