I bought a 25" Amplifone tube once in a S/Wars cab with 1 x pin broken
Normal soldering did not work, so I had nothing to lose. (I wanted to try silver soldering / brazing)
I used a flame torch to see what I could do. As I have welding and soldering experience, I moved the outer peripheral of the flame towards the neck & pins.
Before the hottest part of the flame came near the pins the vacuum discharged and the tube was finito'
Moral of the story
Not really doable, unless the pin is crimp-able or enough is showing to solder
Having said that:
I read a thread by Blaine Locklair?, where he picked away at the glass till he could wind a wire around, and it worked, but looked ugly