Most add-on pockets rely on a waist strap (like your weight belt, or the waist strap on a backplate harness) as the (top) point of attachment. Lots of those on the market. You mention a BCD - is it a jacket style without a waist strap, or a weight-integrated unit, with smaller pockets (and no waist strap)? Another option would be shorts (X-Shorts) with pockets attached, that you can wear over your wetsuit. Try a search on SB for comments on these - most people who try therm really like them. One thread may help get you started (
http://www.scubaboard.com/forums/accessories/247072-new-x-shorts.html) Dive Rite makes a pocket like this, and I have one which I bought on the basis of similar logic to yours. In concept, great idea. In practice, for whatever reason, the DR pocket tends to migrate to the front of the thigh rather than remaining positioned laterally.