I'd be concerned that the alkalinity of the whitewash would etch and possibly weaken the panels. Shade cloth is not that expensive and gives you a lot more flexibility.

They sell greenhouse shade paint. "Kool Ray" is one brand. It's expensive though, and I haven't seen small quantities for sale. Diluted latex works ok as shade paint, but I just learned that it is impossible to pressure-wash it off of poly film. The pressure washer might work better on rigid panels, as long as you are very careful not to break anything.