Backpackers carry water in wide-mouth (2" diameter opening) screw-top bottles that have very good waterproof seals. Nalgene is the brand name, and the Lexan version (less that $8/copy) is considerably tougher than the slightly cheaper polyethylene version. I'm going to try the quart/liter size, and slip it inside a sack sewn from camoflage fabric. I recently filled one with dry scraps of paper, sank it under a foot of water for 12 hours - and it came up bone dry inside! Nalgene also makes little 1 oz. and 2 oz. wide-mouth polyethylene "bottles', and their excellent seals might also make them good choices for micros.
Of course, we always have to rely on visitors to carefully secure the watertight seals before they re-hide them - and that is never a certainty. So I think waterproof log books are a worthwhile idea, and Travel Bugs and trade items that are also impervious to dampness also make sense.