Metal Lathe Accessories has a casting kit for a quick retracting toolpost for threading. It is a dedicated toolpost, but might provide food for thought. As with all of Andy Loftquist's kits, it is well thought out with well drawn, easy to follow plans and excellent castings.
I have both of the multifix holders, which I got for a good price on ebay years ago. They are nice for small threading in reasonable material, but are not very stiff. If you get too aggressive deflection is a problem. I think a sliding dovetail design is the way to go.
Just back out the compound, move the tool back to the beginning (toward the headstock) and feed in as desired to the starting point.
I always start the thread (at the shoulder) with a small undercut, so the tool has a resting point before it starts threading.
My compound is set square to the work, not at 29+ degrees.
The threading dial works as usual for synchronizing the cuts.
One major advantage of the #8 (or #88) tool is a provision for canting the cutter at an angle to match the helix angle of the thread, so no more dragging the tool heal on the thread, or having to severely undercut the tool for clearance, or grinding different tools for different threads.
I posted the same photos in the Graziano forum and there has been some interest there. I very rarely make full plans for my projects which usually just grow out of need and available materials. This is in fact a very simple device but it does require some precise fitting of the slide and the eccentric. Give me a few days and perhaps I will produce a drawing.
Very cool gizmo, look like it could save lots of time. And one less thing to have to pay attention to at a critical moment.
But what I'd really like to see is some kind of stop/throw-out that wouldautomatically retract the tool at the end of the thread, eliminating the need to have a clearance groove there. I've read that experienced operators can do this manually, but it seems potentially disastrous, so I haven't tried it.
the tool is retracted when pulled down and back, to do otherwise seems counter intuitive to me. You can fabricate a self acting screwing machine if you want but it is a very long row to hoe. For my mind just eliminating the concentration needed to return the cross feed index to the zero acurately each time more than halves the time per pass. depending on how fast you run your lathe it is perfectly possible to end a thread in a small drilled hole every time.
The threading stop you show will take care of the problem partly. It is still necessary to break the half nuts or stop the lathe and reverse it. My lathe has forward neutral and reverse instantaneous clutches and with the withdrawing tool this makes for the fastest threading procedure.
When the rolling block is rotated forward as far as the handle is allowed by the flank of the slot, the eccentric pin and connecting rod is placed to be a few degrees over center and therefore a few thousands below the center line of the block where because of the great moment of leverage now in play it firmly resists the slide moving backwards under cutting force.