Physically, the closest you could come to a crown vent would be a "P" trap into a san tee on its back and a street ell into that pointing downward. And even that would not qualify since the flow "upward" from the trap would not have a direct path up into the vent, the way it would with a "true" crown vent. The "P" trap outlet, and the sanitary tee inlet distance together are greater than the 2 pipe size dimension. But I did have an inspector who wanted a short piece of 3" cast iron between the tee and "P" trap for a mop sink basin. It made no difference to him that the trap wier was already 6" from the vent, and adding any pipe would either require moving the vent sideways or the basin would have to be away from the wall since the trap and tee was the exact dimension to put the opening at the center of the MSB.