OBS! NOT ONY ATTNUNED ALLYS ONLY I'M AFRAID!!
Yeasterday when I was doing my onyxia attunment, Jail Break part, right after the dude got his gear; he supply the group with a aura that gives you 65% melee attack speed. At this point me and my two friends who were helping me all DCd (LAN-Party DC is the shit!) and when we logged back in we could all exit the instance still using the buff. We kept it until we logged out.
I kept the aura for only 10-15min, because my friends wanted to finish the quest(we did need it, and they are mage/warlock, so can understand that they didn't care!). So I lost the aura when we redid, and finished it. I can not guarantee any time you will be able to keep it!!
So, how to reenact: 1. Do your ony anntunment up to 'Jail Break'.
2. Follow him until he supplys the aura.
3. Close your Wow for about 2-4min to make sure you DC long enough to that when you log in, you all left grp, failed the quest.
4. You should now be able to keep this aura.
Edit: It seemed I was unclear on that you acctuly can keep the aura outside the instance! I zoned outside just to see if you could, and indeed it I still had it outside.
Edit: Recolored black text on request.
If anything is unclear please ask and I will try to explain. Good luck and happy hunting!