You could try a "hybrid" approach -- have a couple of people carrying shotgun microphones with wireless transmitters, and have them go to near where the audience member is and point it at them from the aisle.
You'd get better unwanted noise rejection than just setting the shotgun mic at the front, and the quality of the sound will be better due to lower room reflections. ("Loudest sound at the mic wins" principle.)
Or you could go really nerdy and have someone with a parabolic mic sitting at the front, pointing it in the direction of the audience member. (NOTE: I haven't tried this!)
