we used water bed mattresses for watering plants in VERY secluded areas with great success. with a gravity-feed method, it's not hard to set up a steady drip system...the only hard part is the initial fill-up. and digging the hole to conceal it was a lot of work too but once done there is very little maintinence needed, if any.

or, you can use a wick method where strips of cotton cloth or cotton rope are put into a water tank and once again gravity will bring water thru the wick and into your plants...i've never used this outdoors but have indoors and it works well but needs testing for proper amount of wicks needed.