Given that there's no info on how to do this, I'll leave the topic up, but it has been reported to Blizzard in case there's a loophole they need to close.
You know, what I don't get is why Blizzard just doesn't leave in 'appearance-modified' pets in the first place, so people don't feel obligated to claw at every possible exploit just to try and get a pet that looks different? It's not a code issue, since people have been using oily wolves or burning pigs or whatever without any real problems. What harm does it do to have a 'sha-touched' tiger when we already have shiny metallic ones in a permanent case of stun-stagger?
In many cases, the appearance isn't even a 'buff/debuff', like the shadow owls in Jade Forest, which as memory serves don't show a buff but nonetheless lose their shadowform when tamed? Personally, I think it'd be a lot easier to weed out the genuine exploits (such as the buff above) if they were separated from appearance changes anyway; the cosmetic effects really have no connection in the code to the game mechanics, as evidenced by the many kinds of both that don't require the other. (buffs with no appearance change, like Ankha, or appearance changes with no combat effect, such as the oil wolf or Lone Hunter's sword)
I think it's more a matter that they need a general solution to ensure every buff is removed or the problem can resurface. Many of them are harmless appearance buffs, but the next one could be a damage or defense buff. So unless they have a general solution that removes all buffs upon taming then they'll have to keep stamping these things out individually, every time.