To be honest I don't think it's possible to automate that. AFAIK C3D knows only the 'average depth' of each pipe, not the actual depth at every point along it.
Maybe someone else will prove me wrong though.
Credit where credit is due! Give kudos or accept as solution whenever you can.
It may not be possible to automate it, but it's possible. trace a polyline over the existing profile and drop it down 5' in the profile view. then trace the pipes below the 5 feet line. form closed polygons and hatch them.
not dynamic but hey it works.