assuming that the EHP did his prep work, and by the looks of your story he did, how about the two slave girls be your gated fiendish huge eye tyrants of doom under a little disguise spell. The beholders AMF is a cone if i recall correctly not a gaze attack, so the pc's shouldn't realize immediatly why their magic is off just because one of the slave girls happens to be looking their way.
Good idea on the beholder disguised as a slave girl... and yes the beholder's Anti-Magic ray is a 150 foot long cone.