I get conflicting reports about this on various forums. If the DLC phones home to check legitmacy, and there is no server to verify, it doesnt' really matter if its already intalled, it won't work.
With DA:O I always thought I had to be logged in (/online) with DLC for it to work. This is because I'd frequently be prevented from loading a saved game and get the message that I had to be logged in because my saved game had DLC in. However after being told that you could play offline with DLC I tested it and was able to play without an internet connection. It has put me off paid for DLC in the future though, knowing that a company might increase DRM via DLC.
Yeah there is some sort of messy thing going on about Borderlands monkeying around with Securom for its DLC. And then there is that sort have to be online DRM thing some of them do, where it, the DRM, does have to phone home but it can be playex X times offline before it is "required" to phone home again. The industry has really made a mess of things in terms of DRM. They're all over the map with it, rarely clear or upfront as to exactly what is going on before the release, and just overall making it more and more unpleasant for customers to deal with.
My favorite part, is where they, Ubisoft, try to pretend it's for our benefit. What BS. If I am that concerned about not using discs, guess what, I have a half dozen choices of digi stores i can go too. They're taking away that choice, and then claim most people wont' care. Hey, most people (the general population) still don't understand whether or not their machines are capable of running a game and flood the forums with help, I can't run the game messages with my IGP that Best Buy told me was "great for games"... that doesn't mean that's the way it should be.