I've got a 16mm bar that'll do nicely.. means I can do it all without repositioning the work in the clamp too, as the travel in the mill is only around 70mm, whereas with your approach I could do the whole lathe bed length... can also fine tune the radius with cutter seating depth.

Just need to build a workpiece clamp on the toolpost, but that's not too big a deal... time to dig out the mig...