Mail Heirloom from Alliance to Horde alt?

Not at this time, Yeahdardy. As transference of Heirloom items may only be completed via our in-game mailing system, the same restrictions will apply. So, no cross-faction or cross-realm transfers. :)

If you feel that we should expand the methods by which Heirloom items may be sent to characters on an account, my friend, please do let our developers and desigers know by posting within our Suggestions Forum.