With the new mechanics of berries in 5th gen, it's impossible for one to reduce the HP EVs in such a way that a pokemon loses enough HP to have it go under zero. However, in black and white 2, they introduced Join Avenue.
In Join Avenue, there are people who set up shops and they can alter a pokemon's EVs both positively and negatively. For instance, the Salon, when fully upgraded, can reduce EVs to 0. the Dojo and Cafe allow you to level up your pokemon like with rare candies and also increase your EVs in each stat by up to 64.
Unless 5th gen patched up allowing you to reduce HP like that with EVs too, I think this glitch is still viable. I have a Salon of level 5 right now, but I don't have the Makeup that reduces HP EVs to zero. If anyone else can help me with this, I'll be so grateful! I really hope they overlooked this.
EDIT: I tried this out on a level 75 Kyurem with 52 HP EVs. Used Slender Make Up (-50 HP EVs) and his base HP did go down, but not his actual HP, which stayed at 6. Sorry about the false alarm, guys