A friend of mine has a Gibson ES135 (modern) brought in for the truss rod repair. The threaded end snapped off, you see it quite often on Gibsons.
The SM rescue kit was used to cut deeper into the neck and cut some treads on the rod. Still, no success, tightening the nut does little and the neck relief is way bigger than acceptable, even jigged to help the rod.
Normally, the next step would be removing the fretboard and replacing the truss rod, but there's another way I read reports of, seems to work in some cases.
Has anyone tryed this: locating the truss rod ancor, removing only a part of the fretboard, milling (?) the ancor and removing the rod through the headstock hole, then replacing it the same manner?
I did one like that 40 years ago, but I don't recall exactly what needed repair. It may have broken at the anchor.
If the rod has been rethreaded, what is the present problem? How do you think cutting off the anchor, pulling it out of the neck and putting it back is going to fix it? How do you plan to reattach the anchor? Aren't those rods in some kind of soft plastic sheath (which would make R&R difficult).
Indeed, it is possible to excavate at the end, drop a threaded steel slug down inside, and screw in a new truss rod, but:
The most important issue is whether having a working truss rod will fix the neck relief issue. From your description, I presume the truss rod is still anchored solidly and that no matter how tight, it still doesn't pull the neck straight.
IF that's the case, then replacing the rod will result in no improvement. On the other hand, if the rod is pulling loose from the anchor, such a replacement will fix the neck.
My best guess is that you'll be better off assuming the rod is working just fine, but not effectively straightening the neck. With that in mind, you might be able to reinvigorate the truss rod action by either re-fretting with tight frets, or by removing and re-gluing the fingerboard. Interestingly enough, that in itself can cure a nonworking rod by making the front of the neck less compressible. Hide glue is the stuff to use because of rigidity.