I suppose it could be done. The main reasons against it would be the surface of the "Bars" being rough and possibly causing damage to your raptor ( more so than the smooth Conduit surface) ; the Outside diameter of rebar is smaller than EMT as well so the distance between bars would need to be smaller; also, rebar is Raw steel it will rust very fast without being treated and painted. Even then the application would need to be frequent. Maybe an epoxy coating would make it sealed and smooth them out a bit. I personally would not take the chance of the surface damaging a bating bird. I would not imagine rebar would be any cheaper than the EMT bars anyhow.
It is good to have an end to Journey towards, but it is the journey that matters in the End. - Ernest Hemingway