Guess the shadow catcher may be used to alter color only, not displacement.
Moreover, after a bit of thinking about it, being able to catch shadows and displace in one single pass would be limiting because you could not cast a real final shadow from a different direction. The shadow catcher should be able to catch only a single light source, not visible in final rendering, used only for the fake shadow displacement.
And i don't think this would be easy to handle, if possible.