Alternatively - on guitars (sometimes made of cedar, in fact) they typically cut a channel and embed a strip of celluloid, polycarbonate, or similar material as a "binding." With a little planning, it seems like it'd be pretty straightforward to use the table saw and cut a small rabbet into the edge of the strip that's going to be the last dark one for the topsides, or first light one for the bottom (or even a bit of each to keep it symmetrical). Then install the strip as usual, complete the hull and then after the rough sanding and fairing is done come back and embed the binding material (which could be anything, really - leather, rope, wire, wood - subject to weight and aesthetic considerations) and then finish as usual.

Not what you are describing here, but I cut slots for decorative strips after the boat was 'planked' when I built one of my 'wood-look' projects.
As I recall, I tacked a guide strip to the boat and then made the first cut with a Dremel saw and guide fence. Then the second cut was made with a different guide fence to set the width of the slot. It seemed a quick solution to the problem of putting a 'curvy' strip into the deck, or adding a strip to the hull if the hull planking didn't follow the line I wanted to emphasize..