A construction trick I've seen in some places for a narrow pit is to pour the concrete such that a heavy steel grate (multiple sections) fits snugly into a channel at the top.

As far as the building inspector knows, its a "drainage area for grease droppings". Should be no safety issues because most of the time the grates will be in place - certainly during inspection. I can see how a building inspector might balk at an open pit in the center of a pad.

Don't know of any installations like this in a home. Maybe someone here has one and can advise.

Ken300D