You guys might want to consider looking for a used portable eye wash station. They are pressure charged, can range from 5 to 15+ gallons. Some only have the eye wash faucet, but many come with a hose and "shower" head.
A few of us used them while out in the field, run some rope and tarp in between a couple of 5-tons for privacy, throw in the water, hooked up the air to the brake line and instant shower. In the winter, you can easily add in some hot water to warm it up. You could even stick the can close to a fire to warm it up, if snow is available you wouldnt even need to worry about water supply. Of course we always towed a water buffalo so supply wasnt too big of a issue for us.
You could also adapt a water fire extinguisher, I've seen many people modify them for an air tank, why not for a shower system?