Noble effort, but I do not see the real usefulness. You cannot turn something that long without using tailstock, steady rest, or follower rest. Either of those totally negate what you are trying to do. For actual usefulness, just use a bar about the max length you would turn something that large without external support. Even then, tool pressure on the part will spring it slightly, and force any play in the headstock bearings and flex in the carriage to also be a contributing factor. I have a Jet 13X40, and just used a 1" ground bar sticking out about 6", and indicated it. I would likely never turn anything much longer and expect it to be straight. Also, Vertical alignment of the head will also have an affect on turning diameter, but to a much smaller extent.
Far more important is tailstock alignment, done with a bar between centers. I have to check that on just about every part that I need high accuracy on, as the lathe must have a very slight twist in the bed. I have a Starrett level, but it is 0.005" per foot, not enough accuracy to adjust any bed twist. I do use a tool post grinder of my own design very occasionally, but still use with part between centers if part is very long.