Inlay Cutter Head for Veritas Large Router Plane

ive got that router but have never tried that !

The accessory is really nice. It's a little finicky but once you get use to it the benefit is nice. 
Check it out here: https://www.leevalley.com/en-ca/shop/tools/hand-tools/planes/maintenance-and-accessories/69876-inlay-cutter-head-for-veritas-router-plane?item=05P3845#share-modal

I wondered the same thing when I saw it.  I wonder if you could make fence/follower to attach to the bottom (a round disk maybe?) that would allow you to follow a curved edge?   They sell a straight fence for it but it has a couple of countersunk holes in the base that would allow you to attach a wood base you make.  Perhaps you could build the follower into a wood base?  Or if you already have their fence accessory, maybe attach a follower to it? 

Just thinking out loud.  

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.