Your apprehensions are valid. The condensation will provide the electrolyte for the galvanic couple. Corrosion of the steel hangers will occur. Then the rust from that corrosion will cause the stainless to corrode also.
 Eliminating the galvanic couple is essential. Isolate the hanger and then protect it from corrosion, so it doesn't contaminate the stainless with casual rust.
 Secondarily, watch for the corrosion resistance of your SS piping. Certain welding and surface treatments can greatly enhance or reduce corrosion resistance.

Thanks for you response Mcguire.

Welding is not an issue; the pipe is manufactured in modules to be connected by rubber sleeves.

The project engineer seems to be convinced that galvanized U-bolts will be OK. My concern is the materials are very far apart. The SS will be cathodic to the steel and even more cathodic to the Zinc, since the cathodic area is much larger than the anodic, it will accelerate the corrosion of the galvanized finish and corrosion will eventually get to the steel.

Or, will the natural zinc oxide passive the process?

The electrolyte will be limited to condensation.

Regards

You are correct on the zinc having to protect both the hanger and the pipe if they are in electrical contact. This will reduce the life span of the zinc coating. It sounds as if the success of the design depends on the quality of the insulation between the hanger and the stainless.