I don't think I would want to create a reference to that object, not what I am trying to do.
I know that is not what you're trying to do, I'm saying that you should do it. I'm assuming that you want to return either a float or a structure in order to not have to write out 'position.x' every time.
So to answer your question, no you cannot. But I don't see why you can't just retrieve the x float from the structure Projectile::ReturnPositition() returns. Is there a reason you don't want to?