You're limited to stock threads so it's utility is not that great for the amount of effort to build such a thing.
On the other hand a tail stock die holder will perform pretty darn good threading that is fairly well centered. Not AS centered as single point cutting. But with a bit of care centered enough. And there's tricks to aid in keeping it centered better. So for "quick and easy and good enough" threading it is a lot more convenient than this crazy rig.
And from a quick check the mini lathes all come with change gears anyway. Just learn to use them. It'll be a lot less trouble then trying to adapt a mini lathe to such a get up.
Keep in mind that the old Unimat and the still current Taig and Sherline do not have any ability to accept change gears or any sort of power lead screw. So they HAD to rely on Rube Goldberg setups of this sort. Interesting to study but well worth avoiding if you are looking at a machine which will perform threading in a more normal way.