A reference for building baritone necks. I know it's basically "extending things", but my initial cursory internet search hasn't turned up much about truss rods (i haven't looked that hard).
I've also only looked at the obvious places for truss rods. stewmac has a "kit" which includes a 24" rod which can be cut and threaded. That sounds promising. I'd rather do that than welding anything.
Carving a longer heel so the neck length to relieve is shorter is not a bad idea either. It would reduce the reach, but I don't spend a lot of time beyond the 15th fret anyway.