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.  

--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.