Deathbringer's Will from ICC will also let you shapeshift in form.

I hope, really really hope, that this is a sign that they may overthink how costumes and forms interact - I mean, even Ghost Wolf and Metamorphosis cancel costumes, the latter is important in PvE combat, and both in PvP.
I wish that shapeshifts would just override costumes, not overwrite them - while shapeshifted, the costume is disabled, but the buff persists and becomes active again when shifting out - but costumes with a short duration and long cooldown, like the Locket (or Frenzyheart Brew!) would display even while shapeshifted.

With a little helper is where my feral tank/dps druid threw up her hands in disgust and stopped trying for the meta. I wasn't going to be able to put together an int set during the holiday and wasn't willing to run around being useless.

I'm actually kind of surprised to see so many people convinced that there's no way for a feral druid to do achievements like Little Helper without respeccing or meleeing in caster form like an idiot... There's actually a very simple way to do it, and it is my preferred way even on other classes...

Use a vehicle. I personally use the cannons up on top of the keep in Isle of Conquest. It gives you a good view of the surrounding area, you can attack and kill enemy players from a great distance for credit towards your achievement, and you're far less likely to be ganked in the first couple minutes before you even get a single HK, even if you have no PvP gear at all. Hell, this past holiday I was doing just that and an enemy player was launched onto the tower to attack the cannons... When they started attacking mine, I didn't even bother getting out. He was melee and stayed close enough that I just aimed down until the cannon was hitting the platform it was sitting on and fired away. Wound up killing the guy.

On my feral druid, I actually got the achievement pretty quickly in Isle of Conquest too, but not using a vehicle. I basically just stayed with most of the group and used stuff like entangling vines to annoy the Alliance. Funnily enough, I never once died - it's like being a gnome just made the Alliance overlook me the entire time. I wish I had thought of a vehicle, though!