> Even in the Metaphysics of Morals, the whole concept of duty would be arbitrary if not in reference to rational beings.

In what sense? There's a sense in which there's no moral law without rational agents, but Kant himself is explicit about the "good will" being entirely independent of intended or obtained ends. In other words, no rational agent needs to benefit (or be harmed) in Kant's system for individuals to be morally blameworthy or praiseworthy.

(See GMS 4:394 for the "jewel" analogy, which is most popularly cited as evidence for Kant's position on this.)