I'd go for the Pelican L1 (Too tired and lazy to hunt down a link now, LOL, so if someone could help with that). Takes four 1.5V button cell batteries. A little obscure as far as a power source goes but should be small enough to fit in a PSK you had in mind. If you wanted to keep the keychain lights in a PSK another thing to try would be to open up the light, and flip the batteries to the opposite terminals so that they don't turn on when pushed.