Let's face it, who has ever used AD to upgrade their white or green companion (well, I did once only to realize it wasn't worth it)? There's no point when you can buy better from AH cheaper. Here's some thoughts I have.
Those tokens should be bound to character or possibly account, and only able to upgrade white/green companions. I think the amount of grinding them to get white companion to blue is fair enough (30 to green and then 60 to blue if I remember right), Binding is to prevent exploits/bot farming.
This would likely need new token, and npc for the quest added to the game but that shouldn't be problem.
You could get started when you get your initial get companion quest at level 16. Those quests could be themed based on the type on companion player has active, like if you have guardian type then the quest could be in dungeon/castle environment, or if you have wizard then it could be more scholarly type. Or it could be randomly picked from instances already ingame (like mithral mine/cult prison etc..) scaled to player level.
Suggestions/critics are welcome, but keep it civilized