Since this thread looks to be still nice and active, I will post my question in here(and it might help the OP as well).
Which of the DLCs can only be visited once? I have read somewhere that a couple of them can only be visited once and once you're there, you're there until it is completed. I just don't want to get stuck somewhere I can't currently handle and I want to make sure I get all the goodies from the places before they are closed to me forever.
Those are two different conditions
You can only play through Operation: Anchorage once. It's a sim, you can't take anything into it with you, but you can't quit it until it's done. You also can't go back into the sim, as far as I can tell, it's the only one-time deal kind of thing
You have to play through the entirety of The Pitt before you can leave, but you are free to return anytime after that.
Point Lookout is 100% you are free to come and go as you please (you just have to keep buying ferry tickets).
Mothership Zeta...you have to play through it entirely before you are able to leave, but there is an NPC that will repair your equipment to 100% condition and there are ways to heal/rest, so it's not as imperative that you bring everything you need with you.
Broken Steel I haven't played yet