Is a roof sprinker a good idea?
Before I remodeled my house to have vents and radiant barriers and lots of insulation, I did put a water misting system on my roof using PVC pipes and drip mist emitters. The mist emitter was helpful because it broke up the water into a fine mist and sent it into the air, giving me evaporative cooling before the water even touched the roof. Also I had some of the emitters overhang the roof to give some cooling to the side of the house.

It all did the job as expected and the areas where the water mist existed received a great deal of cooling. 

However after running the system for the summer, I saw a lot of roof shingle granules in the rain gutters.

The continual flow of water, even though I limited the flow so that most of it evaporated, caused about 10 years worth of wear on my roof shingles. Not a problem for me, I tore the whole house down the next year and built a brand new house. With modern insulation (I did upgrade the attic insulation to beyond code requirements), I almost never even need to use the new AC.

Here is the emitter:
http://www.orbitonline.com/products/Sprinklers/03/13/19/934/