I'd suggest constructing some sort of rube goldberg device to do it for you. At least the effort would be fun to watch.
If you are traveling with it I guess the consensuss (sp?) was hadn tighten. This is for a screw in reg correct?
You could always do the old dry loctite trick. Add a drop of loctite to the threads near the top of the threads closest to the reg (where it would be inside the neck of the tank) and let it dry, then carefully screw the tank on.
That way you don't have to go through the brute force process to get the reg off in the future, but it still won't come off easily.