re: Inlay Cutter Head for Veritas Large Router Plane

This accessory has a brass nipple that screws into the fence rod and allows you to follow a curve. The problem is the travel distance of the fence rod with this nipple attached. A separate wooden fence as you suggest will work if you have a slot in it with a flat head screw. 
Another issue is this works when following an outside curve. However if you have a rectangle workpiece and want a curved inlay it becomes a whole different challenge.