Well, they are surprisingly not watertight. That was the first roadblock that I came across. If you decide to use them as dry storage, make sure you use a liberal amount of teflon tape on the threads to keep them sealed.
You were trying to build as high a compression rate as possible inside the light though...that's vastly more pressure on the o-rings than you would get from a simple dunk in a couple of feet of water. I'm sure that once you get deep enough though, maybe 10 feet or so(?) and the stock seals would not be enough to keep water out.
I wash my MiniMag in a sink under full water pressure with hand soap a few times a week due to the various nasty things it gets exposed to in the normal course of my job and have done so for years with several MMs and have never found moisture inside one of them. I have dunked them in various shallow depths countless times over the last couple of decades never finding moisture inside the light. Of course, I have always kept the o-rings well lubed with silicone grease (or various other lubricants over the years), which I'm sure helps with water resistance, but would be of little value against the high pressures built up inside your fire piston.
I would think that for simple water resistant storage, unless you plan on going diving in fairly deep water, a MM with well lubed o-rings would do a pretty good job...of course, teflon tape definitely wouldn't hurt and may even prove useful for other things in a survival setting. :shrug: