I take it the rack isn't an enclosure? That would protect from most drips like that.
You could try running plastic over the tops of the racks...but I don't know what you'll do to ventilation or what happens if the heat warms the plastic up and releases some kind of gas.
Not to mention if you have fire suppression...blocking things like that from reaching the equipment, what would that do?
I'd probably look at finding a way to run a trough around where the AC leak was and re-routing it. Anything you do near the equipment will affect heating/airflow/fire suppression/safety, and putting things in the ceiling that isn't made for it can be a safety/fire hazard as well. Regular maintenance of the AC unit/inspection of the unit may help mitigate the risk.
Rerouting and stopping at the source would help. Maybe find a way to run a remote water sensor that can alert you if there's an issue?
Also..insurance. Insurance to cover damage. Along with spare equipment should there be an accident.
Summary...get enclosure racks and/or minimize the risk at the source, the AC unit.