Good question, but would you equip a cache and designate it only for certain people, rather than anyone who might be in distress?

The point of some kind of automatic notification is to notify HQ that the cache is being used and some sort of response should be generated, either a rescue or crime scene investigation. Appropriate signs are required obviously.

For that matter, the contents should not be very appealing -MRE's with flameless heaters, tarps, wool blankets, jugs of water, FAK.

I would equip the landing party with an appropriate radio, securely packaged, and a sat phone, likewise packaged (separately). Everyone would have an island map and probably their own cell phone, etc.

In my experience, you begin to get radio coverage once you are about 500 feet high, improving as you ascend. The radios carried include marine channel 16 (emergency and distress) as well as the park freqs.

Solar charging capability is a great idea. Maybe some sort of power bank?

Thiskind of operation, surf landings in the back country will probably be quite common in the future, Doing the ruotine enough times, and there are bound to be incidents and problems.
Geezer in Chief