Vestibules for conditioned buildings

We have a CS project, climatized, on which the client is insisting we should not have entrance vestibules, which to my understanding is mandatory under ASHRAE 90.1. Are there any acceptable alternative options for this requirement, like air curtains or other? We have already checked for exceptions but none of them apply for this project. The other idea is to turn the whole entrance lobby of the building as the vestibule of the entrance, and leave it naturally ventilated (project located in Guatemala City, with a very mild wather during all year).