Depends on the direction said hot air was expelled toward. If from a passenger beside the driver, the hot air would serve to provide a force perpendicular to the direction of intended travel, thereby causing vehicle stability concerns. If, however, the hot air were from the back seat directed toward the driver, the hot air would provide a force directly opposite and counter to the direction of intended travel, thereby causing drag.
The takeaway for Elio drivers? When traveling with an excessively talkative passenger, attach a parabolic or hemispherical dish to the back of the driver's seat and drive with the rear hatch wide open. Although the open hatch will interfere somewhat with the smooth airstream over the car, this increased drag will be far outweighed by the added thrust the pasenger's hot air would provide when properly redirected.