I'm with Pete,
I've seen a couple of people use things they can slide over their tool post for setting exact centre but never for general turning.
In fact and as I've said before, modern lathes toolrests come so that they can't be lowered much past centre height, really only useful for scraping cutting. Which is why I had a couple of toolrests made when I bought my new lathe.
I can see how a rack and pinion might be useful for for some people to adjust the toolrest height but not a static set height, you'd need a different toolpost for each tool!