If I was considering doing a project like this I would want to add a significant amount of compost to the area (organic matter) to help hold water, nutrients and add life to the sandy soil. I would have a soaker style hose to ease the water bill. I would mulch heavily at least 2 to 4 inches over the whole area after planting and plan on adding organic matter every fall for a few years to build and maintain the soil in a happy state for the hibiscus. You might set up something to over head water (to inrease humidy levels) evening and early morning during the hottest part of the year. I would look around Yuma and see if anyone is already doing it and see how they look during this hot part of the year. I would guess your best blooming would be spring until May or so and then back off as the real heat hits and return as you got to the Sept to Dec time frame with may Jan-Feb being a non to minimal bloom period.
I know my Dad has one but I think it is in a pot and whiteflies are his biggest gripe. (They live in Yuma)
My Hibs here in Texas (in pots) don't really like the over 100's but survive it and at least you get to plant yours in the ground, I get to bring them inside from Nov-March each year. If I was there and I wanted hibs, the above procedures is what I would do to have them and expect them overall to be a joy but more work than other xeric options that are available to you in the desert.
Good Luck and Happy Growing David

Thanks for the reply. I am so sick of seeing all the desert Âtype stringy stuff planted around here; but itÂs hard to get anything to grow when itÂs 115. My Hibiscus stopped blooming about a month or so ago  about the time the temp went over 105. I was told, and planted, 3 dark red Hibs that seem to be handling the heat better than the lighter pink ones (2) I also planted. The five plants seem to be doing as well as can be expected under the circumstances. I was watering only once a day, but noticed they would wilt in the heat of the late afternoon. Now I water at 1AM and 1PM. That afternoon watering seems to be helping quit a bit. They donÂt wilt anymore. The soil is very sandy all the way to china, so over watering is not going to be a problem.

I find your misting idea intriquing. I had actually thought about doing that  putting a misting system in place along the top of the wall behind them.