How can a long spiral be accomplished on a lathe.

[Magicniner]

No Sir, did not forget. No dividing head or indexer needed.

Cool, eventually you'll mention all the required bits thus giving us a good clue as to that to which you allude

[Carm]

"required bits"

Something (say a shaft) to allow the work to be held on a center above the mill table. The height exceeds half the diameter of the chuck or drive dog radius that holds and turns the work.
A "tailstock" to support work exceeding the usual Dx3 rule. Same center height.
A sprocket or pulley on the end of the shaft.
A sprocket or pulley attached to the x axis leadscrew.
Chain or belt to connect the two. Possible toothed belt and possible take up idler. Can of course be geared.
Simple multi starts can be done by slipping teeth/belt on the sprocket, or swapping dog drive off a three jaw, making a shot pin index for the drive dog, etc.
You'll have to figure ratio for your mill. My Bport has a 5 pitch, the old K&T had a 4.(also provision and gears for leadscrew)

I couldn't find Jim Glass' picture, hope this illustrates.

[Sunset Machine]

What is the lead? It looks multi-start too. I've turned a .625" lead on a Logan, but this appears to be a bit more. Your lathe will become more of a shaper; it's very stressful on it. Go very slow with the RPM else the gearbox will be screaming. Ivan Law's book has an easy section on change gears for this if I recall. The depth of cut is small, looks lathe do-able to me. Actually looks like fun.

[mcostello]

Shaft would be lots easier than making a nut.