This was my thinking in the other thread, and my meaning about "in" the concrete. Essentially soaking the sensitizer into the top layer. (Might not work.)
If a way can be found to exploit the ferric chemistry of concrete into a natural sensitized layer then it is part of the surface, not just sitting on top of the surface.
Of course, there is a real possibility that the other reactions going on to set the concrete would wreck any light sensitivity. But it sure sounds worth trying.