Welcome to the Ericson group!
The enclosed areas behind each settee are pretty full of Very Heavy fiberglass layups for the shroud load-carrying rods. There is very little storage potential lost there. We have similar settee seat backs in our boat, and I removed the teak plugs over all of the screws many years ago to "check it out" as it were.
(Matter of fact I had the port side one off again last week to run a new wire to the panel, which is on the port side...)
Inside of those you will find the lower end of the Navtec SS rods threaded into some large solid bar stock aluminum that is held in place by recesses in the moldings. It is a Very strong system.
As for other places, EY used to set the screws flush in the panels that covered some areas with tanks, and then gel coat over the fastenings. Tedious to remove the screws if you ever have to replace or repair a tank, but do-able with patience.
Full disclosure - other than some offshore crewing on an E-32-3, I have not worked on one. At least not to the extent of removing fastenings.