Ask Mr. Smarty Plants

We think you're looking for halophytes (salt-tolerant plants) rather than epiphytes.  You are right that epiphytes derive most, if not all, of their nutrition and water from the air rather than through the roots.  The roots of epiphytic plants are often of use solely as anchors to hold the plants to some substrate such as the branch of a tree or a rock.  We know of no trees or shrubs that derive all of their water from the atmosphere in the conditions you describe.  However, there are some rain forest ephiphytic trees, strangler fig (various species) being a good example.

You do not say exactly where you're located other than to say that you're on the Arabian Gulf.  However, the environmental conditions you describe are not all that unusual in many parts of the world.  Since the extent of our research and the limits of our expertise are limited to those plants native to North America, we really don't think we can give you more specific recommendations.  We do not have sufficient information to give you an opinion about the adaptability of Sisyphus spina-cristi to the conditions you describe.

Our general advice, though, would be to look for plants already growing in or on the periphery of the area about which you're concerned since these plants will be best adapted to the growing conditions in that place.

A University of Karachi organization, The Institute for Sustainable Halophyte Utilization, might be of some help to you.