Having an aluminum receiver tray is just so much extra useless weight. I have not found a manufacturer yet who has made one, and the reason might be what Skela mentioned or it might be the fact that an aluminum one is kind of overkill.
To answer the interference questions...no, you shouldn't get any unreasonable interference from a metal servo/receiver tray. The only time that would be a concern, would be if you have bare wires or some kind of metal to metal contact.
But, to satisfy your need for an all aluminum Savage....if I come across one, I'll post the link.