What I would think of doing is cut the thread from the headstock end
toward the tailstock end, using no tailstock but supporting the long
unthreaded part with a center rest and other outboard expedients. I
would start by cutting a doot of thread normally and flipping the screw
over and screwing it into a special 'nut' in the lathe jaws.

I would make a long tight fitting nut and slit one side and
semi-permanently (glue?) it to the chuck so it could not move. Then
screw in the just threaded part and clamp down and cut threads away from
the chuck. When you need to move the screw just loosen the chuck jaws
and screw in the section you just threaded.

There might be a small discontinuity where you flipped the screw, but
that could be cut off later.

--
Free men own guns, slaves don't
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