The obvious question is whether or not that is necessary. I completely understand the idea that sealing all of the surfaces has the potential to protect these otherwise hidden areas, but from my experiences, there doesn't seem to be an absolute need. Cedar is used on ribs and planking not only because its light weight and workability, but also because it is extremely rot resistant.
I have pulled original ribs and planking from hulls over 120 years old and found that the wood is absolutely perfect in that unvarnished area behind the ribs and under the planks. The only places where I have seen rot are where water can stand and collect without draining or where a boat has been in contact with the ground or leaves and water.
A few good coats of whatever hull concoction you use seems to be more than enough. My mix is a blend of BLO, Mineral Spirits and Turpentine applied warm (nearly hot). It penetrates, seals and hardens.
What I would definitely varnish on the backside are the rails. Many builders left the backs of the rails unfinished...