Dik:
Many of the problems I’ve seen and heard about with these ceiling systems is the deterioration of the structural and ceiling hanging materials in the humid space btwn. the ceiling plane and the actual structural floor/roof above. At the least these spaces should be vented to control the humidity and the damage it does. Maybe this space should actually be lightly pressurized with less humid outside air. That way, the air flow would be out of this ceiling space and into the generator room; rather than humid air being drawn into this ceiling space. I can imagine that the high humidity would eventually have adverse affect on a plaster and metal lath ceiling, particularly since it is almost constant and likely acting from both sides.
Why not really be different, and make this ceiling space be a conditioned space, so as to control the heat and humidity in that volume. Design a fairly typical hung ceiling system, but on its underside apply a good vapor barrier material which is actually sealed to the exterior walls and around penetrations, etc. Provide some controllable ventilating registers or slots in the ceiling and into the generator room. I have no particular vapor barrier material or manufacturer in mind, maybe a 20mil EPDM, just glued to the main ceiling material. I don’t have the details worked out, but I imagine this as an upside down roof system, to keep the humidity out of the ceiling space. Your friendly Mechanical Engineer (HVAC) might help you with this.