Good idea on looking for a parts diagram. Mine is a 2005 Base and I found one.
Just as you (jolt) mentioned, there is the visible outer cover over an absorber and then there is an impact bar (assume it would be metal). So I would have to drill at least through the foam absorber to push in the sensors heads and see if there will be room to snake the wires through the absorber and around that metal impact bar (or perhaps change bits and drill through the metal).
It's hard to think about drilling that first test hole in the bumper because there will be no going back if it becomes more difficult than expected. If anyone else has something to add to the conversation please do so as I'll wait a few weeks before deciding on this one.