A warrant to search a house might well cover a small bag in a closet. It depends on what the warrant was issued to search for. If there was probable cause to think a person might be held against their will, then looking into the closet would be legal but not opening a bag. If it was a search for something small, like drugs or stolen jewelry, opening the bag would be legal. I don't know what you're getting at here.
My point is that warrants are flexible. If the cops have a warrant to search a room you can't get out of it by arguing technicalities. The legal system is run by humans, not computers, so the guy who thinks of a clever way to reclassify his data has only made it slightly more expensive for the government to take it, and then guaranteed he'll get fined.
Many, many computer geeks see that the legal system is basically a series of algorithms, conflate that with the algorithms running their non-human computers, and think they can figure out a way to hack the algorithm. That may be the case (see Mitt Romney's tax burden), nine times out of ten the humans running the Court system will see a clever hack like this as bullshit intended to keep them from doing their very important (and they think they are very very very important) jobs.
If you find two cases of this sort of thing happening in the US, let me know. I know about Lavabit, but they taunted the happy fun court system, generally a mistake. One analogy might be retained email, and the courts have decided that they can't require a company to come up with destroyed emails, or penalize them for not keeping emails past their policy-defined retention date.
Microsoft is gonna be a case of this real soon now. There won;t be very many public examples, because generally to be a public example you'd have to a) publicly proclaim you were using some legal stratagem to keep your user's data safe, and then b) publicly admit it didn't work.
Part of the problem with Twitter's strategy is they publicly announced it. If you're a Judge who thinks you are all that stands between Civilization and Anarchy, with your Fair Rulings sending Bad People away; you are not gonna appreciate that a multi-billion company has tried to make your rulings harder to enforce. When the DEA asks for a warrant to search some guy's private messages on Twitter, and you think they're right, you are not gonna be in the mood to rule that the legal system has been successfully hacked by an MBA and a couple engineers at Twitter.
Twitter are really going to have to prove that they cannot access the Irish data, and you aren't going to be taking their word for it when they say that [insert database feature you never learned about in law school because law school doesn't teach database theory] prevents them from accessing their Irish database.