Most of you have probably all ready thought of this, but here goes.
Today I needed a 1/16 radius on a 1/8 lathe tool.. So here is how I did it.
I put a Dremel flex shaft thru the lathe spindle and clamped in the lathe chuck. Then I made a stand for my ball turning tool, milled for a 5 degree relief angle and slotted for the 1/8 tool. It may not be perfect, and it was slow going, but it sure did a good job.