I've recently designed a GPS based vehicle locator/safety system for railway use. It uses 3G, Data Radio, voice radio, and satellite interchangeably to report to the control centre and receive alarms and alerts in return.
In my experience satellite is not really expensive. You can buy packages with a per message cost. I can't see how a per-message side-channel on existing aircraft satellite transponders would cost any serious amount at all - especially if it was only used for emergencies.
Asides from that, there is always the option of using normal VHF/UHF radio to squawk current location. Again, only in emergency, but sure to be recorded by any aircraft and ground-stations in range. It could be turned on for instance if the radio was set to the emergency frequency and would append a 'selcall' or DTMF burst containing GPS coordinates at the end of any message. Many land-mobile radios have this feature already.
Last edited by Mahatma Kote; 28th Dec 2014 at 23:16.