Thanks for the tip, but no, the screw has just loosened as evidenced by the slots registration (maybe 1/4 turns or less). Unfortunately the loose side plates caused a small, shallow chip in the wood right at the edge of the plate on the side opposite the screw slot.

I've found a fairly comprehensive screwdriver set (hollow ground) at Midway for $65 (includes 54 flat bits). Hopefully one will fit.