with the heat we're having, i'm not sure how you'd control the temperature. i have a 3000g pond, surface 80% covered with lilies, and the temp hasn't been below 85 in weeks. i just have goldfish and small koi, which seem to tolerate the higher temp's ok, but not sure how cichlids would do. i'm guessing that in a couple hundred gallon pond the temp fluctuations could be pretty dramatic, particularly if it was out in the sun.
on the other hand, if you need lilies to help shade the surface, let me know -- i have plenty i can give you.