I don't think containing a pyrophoric substance by itself is a good idea, as the slightest perforation or abnormal situation would make the animal explode or cause a big wound at least. Not very likely to pass the Darwin test.
However, there is already an animal that exists (passed the Darwin test) and does something very similar to what Dragons do. The Bombardier Beetle has two container in the back of their body, one holding hydroquinones and the other hydrogen peroxide. When these chemicals combine, they create an exotermic reaction that is used by the beetle to defend itself from predators.
So, your dragon doesn't need to have an internal container with a peryphoric substance, it can have two (or more) containers holding inert materials that, when combined, create a pyrophoric substance.