Basically, yet another creative use of the "Demonic Circle: Teleport" ability.
What you want to do, is right before the Alliance potentially capture the orb and the keep door is knocked down, place a Demonic Circle just inside where the door is.
After that, as you're on the offensive, try not to die, or else your efforts will be in vain (although warlocks are prone to dying as of late). By placing the Demonic Circle inside the keep, and not dying throughout the offensive portion of the game, you may teleport through the keep door once the yellow gate is down. PLEASE NOTE YOU ONLY HAVE SIX MINUTES DURING YOUR OFFENSIVE ROUND TO DO THIS.
Now although it seems like this is a lot of work, often in non-premade battlegrounds you'll find yourself running tanks and seaforiums at the keep door to no avail for half the round, and this could potentially save a lot of time.
Edit: Please note you DO NOT need to destroy the final gate to click the orb.