Alyssa, a larger fern has more leaves absorbing water and nutrients from the air. They really don't take in much through their roots at all. The roots just hold the plant upright, and in place. So what you water the medium with or how often you do it is not really relevant to the plant's health.
Mine, up in the oak tree, gets nothing but rain and what it collects from the air. In our dry hot fall and spring weather, we can easily go 6 weeks without rain but that huge staghorn never wilts or looks unhappy. In summer it can handle a deluge every day also. It takes in what it needs through the leaves and sheds the rest.
Spraying the foliage with rain water will keep most any staghorn pretty happy, jmho. You could put it in a pot or on a log, or in the crutch of a tree and it would be happy with sprayed leaves. It really doesn't need soil, pot or any other medium except to hold it in position for display purposes.
