I do not think Section 304.5.3.1 will not work either as there is no low opening. However, there is no high wall opening either; the ceiling would be removed and airflow would be vertical, not horizontal.
It seems more suitable, to me, to consider Section 304.5.3.2 Combining spaces in different stories as the ceiling in the janitor's closet (water heater location) would be removed and the 4+ ft mechanical space above the ceiling would be the space from which combustion air is drawn. The ceiling space is part of the same story, not another story, so perhaps this section does not apply. The commentary is silent about utilizing the ceiling space.
I am hesitant about utilizing the ceiling space. As classicT pointed out, this could be a plenum and part of me wonders if the wiring and piping above the ceiling would then need to be plenum rated.
I had suggested water heaters with combustion air intake capabilities. The GC/Architect countered by asking me about utilizing the ceiling space for combustion air.
The building is a car dealership. The water heater is in a janitor closet accessed from an unenclosed corridor/hallway/passageway that serves the janitor closet and two toilet rooms. The corridor is open on both ends. Its only function appears to be that it hides these three rooms from the showroom. I could theoretically place grilles high and low into this corridor, but then the IBC and IMC prohibit using a corridor as a supply, return, exhaust, relief or ventilation ducts unless outside air is supplied to the corridor at a rate greater than makeup air taken from the corridor.
Only 3,300 cu ft of space is required for combustion air. The volume above the ceiling provides more than this amount. The showroom easily exceeds 120,000 cu ft, but the janitor closet is not immediately adjacent to the showroom.
I may suggest a small 25 cfm fan with ductwork to take air from the showroom and dump it into the janitor closet when the water heater burner is active.