I had a phone call the other day from another guitar tech looking for a way to get a stripped nut off of a Takamine. I came up with the idea of treating it like a pin chuck or mandrel. A pin chuck is used by pen turners (and machinists) that want to be able to turn a closed tube without a tailstock. A mandrel that is slightly smaller than the diameter of the tube and long enough to go into the tube and chuck, is used.
That's a good one!