Try Setting Cast 2" and Copper 2.25", think they may be some what your looking for?
This is just one example,
But even with that setting it may place copper when cast-iron is required.
I think by adding the ability to do this in constraints it would eliminate the possibility of using the wrong stuff.
Now yes, there would have to be another option to "include unrestricted" or something of that sort.
I think it would be really beneficial, at least for myself... (maybe I'm alone?)