Interesting idea. I don't have any suggestions yet, but I like the approach... Make containers you're going to be carrying anyway serve more than one purpose.

Someone on here just recently mentioned gluing lighter flints to a key blank for a stealthy, light weight fire steel. Maybe you could glue a slice from a larger fire steel to the outside or something.

Anyone know if you can get firesteel metal in rectangular blanks? I could glue flat plates of the stuff to all kinds of things...

Edit:

About the only other potentially useful items I can think of that might fit in such a small space... information and money. You could write down emergency contact info, calling card numbers, passwords, pins, etc. on small piece of paper and fold it around the batteries. The tender quick tabs for the sparklite fire starter (any tinder, really, I guess) can be squished into some odd spaces. Money and tinder also have the advantage that they probably won't get a second look from an xray person at an airport.