Was actually thinking about a sleeve with lock screws. Kinda like a heliocoil thread repair kit.
What about taking the spindle to a local machine shop and having them cut it down in diameter a touch, thread it, mount a sleeve, and rethread it to 1 1/8 or 1/4 8tip? You're torque is kinda self limiting by how much pressure you can take with the tool so I don't see this stripping out a nice, 1/4" steel sleeve. And you're gonna keep at least 7/8 of your spindle with minimal extention so lateral torque shouldn't be a huge problem unless you're turning huge and unbalance pieces hanging out in the air without a tailstock.
Or is this just waaaay to complicated a way to go?
Dietrich