I've never run across a way to replace it, and thankfully have never needed to. I have cut and resoldered a focus voltage lead because i had no other option. I think those are ~8kV? I used four layers of 600V heatshrink and it worked fine. The flyback lead is another level above. In theory what you suggest should work, you could even work out some silicone potting, just be prepared for it to fail when you turn it on and be ready to turn it off again. And if it appears to work, run your discharge lead over the outside of the cable and the join while the CRT is powered on to make doubly sure - making sure to protect yourself, of course.