Ok, I don't play a Warlock but my guild master does. For the sheer curiosity we decided to test this out during the boss fight. The move he used is that which allows the warlock to teleport back to the place where he dropped the summon.
This summon lasts for 6 minutes, which is long enough to do the fight and then some.
The boss is Gothik the Harvester, and for those who know the fight, you have two sides. A live side and dead side. As you kill mobs on one side, they end up on the other side. Between the two sides is a closed gate that will not open until 35% on Gothik.
Depending on which side you wish your warlock to be at the end, drop the portal on that side. Let them dps their heart out and when they are needed on the other side, they are able to teleport over.
This includes with the gate closed! (I don't have a warlock so I can't do this in a fraps'd video for you as I normally would.) Anyone is able to confirm if they want.
TL;DR
You can use the warlock summon/teleport to go through the Gothik Gate during the fight.