We fell ever so narrowly short of the goal, but it is possible. Here's the highlight/lowlight of our night, a horribly grainy clip from one of my many many Fraps:
There was another 4% failure, and more sub-30% than I can count. If we'd had world buffs, he'd have died. Or, hell, if we hadn't been retards and lost our Darkmoon Faire buff right away in the zone, he'd have died. Or if we'd prepared with a handful of other unusual consumables ahead of time....
I don't want to give away too much, really, because there's still a huge challenge out there for a group of 20 insane enough to try, and I'm sure there are people looking for things to fill their time post-Naxx who would be interested.
Yes, you need to raid-stack. Yes, you need to do some weird stuff with raid setup and positioning in general. There's a ton of theorycraft to it.
With all 5 priests up, Hakkar's damage and health are multplied, bringing him to well over 1,000,000hp, and makes him do things like this to a Dreadnaught-geared tank:
He has five Aspects thanks to the priests, including an AoE silence that does heavy physical damage, an AoE poison, a stun, a complete aggro wipe, and a 250% attack speed Frenzy every 15 seconds.
10/31 00:44:33.859 Hakkar hits Tehax for 2581. (crushing)
It's not a fair fight, nor is it balanced, but it's fun, and it's hard, and requires a lot of thinking outside the box. It's basically a full-fledged tier 3 20-man encounter. Give it a try sometime -- I'm curious to hear how others find it.
I'm admittedly disappointed that we fell short, and it would've been cool to accomplish this before the upcoming skill/talent changes, but it was a hell of a lot of fun trying.
(Also, thanks to Cryect for a really customizable and all-around excellent mod in IMBA -- modifying the Hakkar module to account for all of his abilities took me all of ten minutes and worked on the first try.)