Thanks yet again BB - my list of possible projects gets ever longer!
That all looks practical and possible. Maybe a bit noisy, but it would also be possible to set up a v-cutter, held in the toolpost, to cut a thread the way you would with a screw box. The big advantage would be that, with the lead screw controlling the rate of advance, you could take successively deeper cuts. You can't do that with a box, as you need a section of complete thread to engage with the existing thread at the back of the box.
Food for thought indeed!