Rick has written an excellent comment about the previous post - please take the time to look at it, as the idea is very good indeed. I have thought of a similar system, but Rick's is the best idea yet.
Having sailed in a few small boats myself, I concur very strongly with Ross regarding side benches. If the seaway gets a bit active, they get in the way very quickly. If one really MUST have them, they should always be removable, but latched down with buttons so they don't come loose when things get really rough, as sooner or later they will if you sail much.
As for me, same as Ross; sitting low in the bottom of the boat with back against the coaming, or sitting on the windward washboard (side deck) is the way to sail a small boat.
For sleeping with a decent amount of dry, level room in a small open boat, one you won't roll off of, why not build in a pair of risers just above the height of the thwart and the sternsheets, with buttons on the backside of each, facing outward to the hull's planking? Then when you want to sleep aboard, you unroll your strong canvas trampoline, slip its grommets over the buttons, stretching it between the risers, and sleep on that up out of bilgewater and high enough above the thwart and sternsheets to avoid hard corners in your back. On some boats that might create a space wide enough for two to sleep.
No need for the obstruction of side benches at all, and the risers, if made a bit wide in their vertical plane and thought out carefully, notched into half frames, should make a good backrest supplementing the coaming or washboard carlin each side. Adds a bit of weight, but I'll wager that it'll be a lot less than the side benches, with no obstruction of internal space and no loose hunks of wood coming loose on you at the worst possible time.
Also, Geoff Leedham has emailed me to say that there is no obvious support for the seats when in the centre location. The cleats for the centre location hadn't been installed at the time the photos of the seats in the "side seat" location were taken. However, in the shot where you can see the seats butted together in the centre "sleeping flat" position, there are 19mm x 19mm (3/4" x 3/4") cleats screwed to the forward face of the stern sheets, and to the aft face of the main thwart. Look at this photo and you can see the outer cleats - there are similar ones under the "sleeping flat" set-up on the centreline. It is a very simple arrangement.