This is me on a mattress of 18 12" balloons inflated to 6-8". I Covered them in a quilt to simulate a "case." I literally belly flopped on this and stood up and jumped on it several times. None of the balloons ever popped until I decided to try closing them with twist ties. Even then it was much more difficult to grind the sharp edge of a twist tie into one enough to puncture it. It wasn't a pop when it did, just a slow deflation. Half inflated like this they are very difficult to deflate. All 18 balloons I used came up to 1.5ish oz. I didn't weigh them on a proper scale, but against a known weight (my pocket rocket).
I laid on it like this, but they went everywhere:

So… I covered them in a blanket envelope:

Here's me on it. I had just fell back on it hard. No pops. These things are really tough to pop. Only a field trial will tell how awesome this is.

My wife showed me that if when I tie them I insert a tiny length of string through the knot, I can undo the knot later very easily and reuse the balloons a few times before they are worn out. It is really easy to do and works pretty well, though I will have to play with a better design. Maybe someone has already come up with a temporary balloon closer? If so, it means you can take out the sharp twist ties and paper clips.
I found that three balloons wide for the mattress is extremely generous. I will probably only do them 2 wide. As you can see this is crazy, but it works. AND IT'S ULTRALIGHT. The great thing is, repairs are a cinch and the repair kit is just as light weight as any other.
All I have to do now is start making "mattress cases" out of cuben or pertex and use them in field trials. What do you guys think?