Never done it, nor seen it done...
A straight knurl somehow attached to a ball/radius turner? If you started on the parallel/cylindrical section, it would probably track the initial knurl if you slowly advanced around the curve - would be quite a force on the ball turner though, knurling's hard work!
What springs to mind is where a ball/radius turner uses e.g. an inserted tool carried in a holder, a new holder with a slot in the end and a knurl wheel carried on a hardened pin through the "fork" so the middle of the knurl teeth is square to a line through the turner's pivot and the pivot is aligned with the centre of the radius you want to knurl - a tricky setup... But probably how I'd do it if I really had to!
Dave H. (the other one)