Restharrow & frank,
There are often such hidden gems of potential large and small storage areas in boats that are not utilized by the manufacturer for cost or other reasons.
However, one must always be careful not to adversely interfere with the STRUCTURAL integrity (strength) of the hull or standing rigging. And, as Steve points out, go slow and check first (small hole, light, & mirror or small hand hole and feel) for what might be hidden behind that closed-in space, lest damage be done to something in the process of gaining access to the potential treasure trove of additional storage area that lies hidden behind.
Seat backs are a relatively safe area. My 1990 Island Packet 35 had seat back cushions stapled to 3/8" ply with 4" protrusion at bottom which rested snugly up against a teak ply bulkhead along the side and were held securely in place by the bottom cushions. Guess what, that bulkhead ALREADY HAD NICE LARGE HOLES WITH ROUNDED EDGES CUT INTO THEM FOR STORAGE BEHIND with a molded smooth fiberglass bottom. It was definitely a very well made quality boat. We stored a lot of stuff in there when cruising extensively and living aboard.