I know of nothing in the NEC that forbids it. If it's not forbidden it is permitted. I've done installations that include many strobes mounted in WireMold boxes and fed by by W/M 500 surface raceway. I've also used 1/2" EMT and 4x4 boxes with plaster rings. The only thing to be concerned about is the height. It may be specified in NFPA 72 National Fire Alarm Code. My (old 2007 edition) says in 7.5.4.1 between 80 and 96 inches above finished floor, and in 7.5.4.2 when the ceiling is too low, 6 inches below the ceiling. Building codes may say other wise.
