However, what is to stop heirlooms from having an achievement-like flag on them? There are plenty of ways I imagine to make it so that upon buying an heirloom a switch is flipped basically saying "You have access to this heirloom so you can now get it for free everywhere."
Thinking about it, the new cross-realm/faction pet/mount UIs would be perfect for this. Shape that to allow item distribution, make the heirloom items you buy at the vendors "On use:" items that when used "learn" the heirloom (just like pets and mounts), allowing you to drag one out of the heirloom window on any character who needed it.
This would solve cross-realm heirlooms and allow you to conveniently have more than one copy of an heirloom, something I imagine many of us have wished for while playing heirloom hot potato between alts.