Does anyone know he we will be able to return to the dlc once we leave?

Not yet as far as i know but i would certainly like to.

Would have been great in dead money so we could search for any weapon mods we missed.

There's an achievement that worries me about this: http://fallout.wikia.com/wiki/May_my_Hand_Forget_its_Skill

It's requirement is: Evacuate Zion.

Since you are supposed to determine the fate of Zion, it may very well be that you don't have to Evacuate the place. Which of course means you won't get the Perk. However even if you do, it does not have to eliminate a return to the place. I guess we will have to see.