Yeah - I've been thinking about the same thing. For my larger kits, no problem: I've included a Camelback bag, roasting bags for backup, and so on.

In my PSK, I ended up wrapping everything in a 1-quart ziploc bag, which besides keeping my "modules" further protected and together, provides a mixer bag for the water purification. I have two Gerber bags in the kit that seem like they'd be sturdier and easier to carry in pockets, and which I'd fill from the ziploc bag.

Barring that, I figured I'd cut or chop up a tablet and eyeball a pinch equivalent to 1/3 of the tablet and dump it into the Gerber bag with untreated water. Even half a tablet in each of the two Gerber bags is probably fine. Might not taste great, but wouldn't hurt anyone.