I genuinely love this idea tbh. Having a melee dps spec adds literally nothing to the class when range dps is more desired, but having a tank spec could be very useful and the spec lends itself to the idea very well as mentioned.
I think it'd be hard to balance against the other tanks if you let the pet fetch aggro though, but it could definitely be good to fetch adds, or to interrupt enemies, and maybe have a damage debuff so you need to anticipate which enemy will be the most dangerous... yeah there's a lot of fun stuff that could be done with it.
Darnit, now I really want this. :Z