John in NM
I would bet there would be huge amounts of chatter and general difficulties with that much cut length.

Even the copy lathes I've seen use a regular tool insert and some sort of mechanism to ride against a master form. Many turners find it only takes a little practice to get them to come out close enough by eye and a few measured points. Open end wrenches make good quickie calipers for small diameters. Larger diameters I will sometimes make a fixed caliper out of 1/4" plywood.