I would think a old bent trailer axle half would make a perfect lathe post. I am thinking mounted verticaly with a adjustable "tool" holder mounted on arm off of the axle tube. the holder would be adjustable both on the radius of the wheel and of course the height, this way any wheel diameter could fit if the bolt pattern was the same, or adapters could be made for differant wheels. chuck up the torch, or plasma and spin the wheel manaully. insert the new piece and using magnetic base indicator check for runout both ways, tack it and true it, weld and done.
After typing this post I decided I wanted to draw it out for my own future referance and I thought I would share it with you for ideas. The more I think about the more I like the idea of building one of these. I have done some old school VDub wheels before on some low buck sand rails and this would have been allot of help.