I think I've figured out how the retention system worked. If you look closely at the area circled in this shot from a 2002 Sport Rocketry mag issue, there is an L-shaped piece of music wire attached to the wing. A burn string passes over it and holds the wings folded back. Very simple but effective method. I tried it on my old prototype and it worked well; routed thread from the motor pod burn holes back down over the music wire hold-downs, then back over to the other side of the fuselage through a wire loop. It holds the wings in place securely.
I'm definitely going to use this method on my version #3 (version #2 which I just finished uses a more intricate method with more fiddly bits).