Not sure how to do it in mantra, but in redshift I can make a material that receives GI and shadows (and I can also disable one or both of those if I want), but not normal lighting. That way my texture matches the scene but can also pick up shadows in case a shadow is cast over it. If you can't do that in mantra it would be possible to render the geometry top faces again but with a shadow catcher material, then in compositing you comp the shadow back on the top faces as you said.