Do you have any pictures of this. And what size round over do you use on your banjo necks. I too am a luthier. I want to build a set up to do what you are doing but for myHi everybody. I wanted to tell you about a rig I came up with. I had an old router motor which I slipped into a hole drilled to fit it in a piece of 2x6... this 2x6 (horizontal) is in turn mounted to a vertical 2x8 which in turn is mounted to the benchtop with a couple of shelf brackets. The hole was split in the front so I could clamp the router motor with a cross drilled carraige bolt. The motor had a ring type height adjustment so I use that to make fine adjustments in cutter height off the bench. I just turn the ring till its the right height off the deck, then clamp the motor with the carraige bolt. Pretty simple really.
I came up with the idea while trying to round the underside of some banjo necks I'm building. I use a large rounding over bit with a pilot bearing, but the problem comes when i get close to the 'peghead' which is at an upward angle of about 10 deg. The router base rides up on this peghead and ruins the cut. With this rig theres nothing in the way so I get a clean cut clear up to where the peghead angle begins. Wish I could show you a picture, but you probably get the idea.
Oh yeah... I had all this junk on hand, so it cost me nothing.