Securing the piece would be fairly simple using the cam-style work-holding jigs pictured here with many posts - essentially, little circles of wood with an offset pivot holes that are screwed to a base with a matrix of holes with T-nuts.
Doing the lengthwise arc is the challenge. You might attach an auxiliary sub-base with outriggers that would follow the arc of a jig external to the piece, allowing the fixed-base router to "plunge" along the arc.
If you have a bunch of these to do, building the jigs might be worth the effort. Otherwise, I'd just do it by hand with carving chisels.