Played around more with the potbox & the curtis and figured out how to get them talking. Just experimented with the controller pot connections until I found a way to make it work as a 2-wire, rather than 3-wire setup.
EDIT: of course the incentive to figure it out was the cost of a replacement pot that would have been the "easy" solution: $29 US. That's pretty spendy for a pot. It sure is a lot of work not spending money!
Blah blah. All of which is to say this bizarre chimera is another step closer to ... something.
Getting this hooked up to the car's throttle cable is my next priority, because I want to try driving it sooner rather than later.
forklift potbox details.