Jdurg--are you thinking of gallium? I wasn't aware that Al formed an amalgam that quickly. Perhaps it's just not that soluble, I can't remember. I do know that Ga eats through it like a beast, though.
OK, why would you use a nitrogen trihalide? That just seems really dumb and expensive. I guess you're in britian so you're stuck with all the BS laws there, but here in America you can just buy a damn shaped charge and blow the door off its hinges. If you have to make your own stuff, it's ridiculous to make something as unstable as the stuff you're talking about anyway.
Next, corvetteaholic is right: get redneck on the door. Here's how:
1. Drill hole in door (very easy considering Al is softer than steel)
2. Insert your favorite hook/device.
3. Attach hook to large chain.
4. Attach chain to your full-size pickup truck's frame.
5. Depress gas pedal and watch as your magnum V8 uses it's full 400 ft-lbs of torque to tear the door, the doorframe, and half of the face of the building down.
This DOES work. I've seen someone with a medium sized truck pull a LARGE TREE from the ground. Unfortunately, I think most of the cars in the UK have engines that are closer to my lawnmower than something that could pull your door out.
Of course, you could shoot it open, too, but that's also illegal. Hmm. Why not just use a bolt cutters to cut off the current lock and replace it with yours?
This is all so ridiculous, and of course, I'm just feeding the troll, but it's funny as hell anyway. If anyone's actually reading this, don't take too much of it seriously.
BTW, if you use and of the Cl2, thermite, cutting torch, etc., you'll have problems with fire and heating the frame/house until it burns down. Just do like dumb criminals do and use brute force. It will work.