RFID on humans, is RFID the right method?
No, it can't be done. RFIDs work in the "near field", which is the area around an antenna where the transmitter can sense changes in signal absorption.  

Low frequency RFID (125 kHz) works at distances in the centimeter range. High frequency RFID (13 MHz) is used up to a couple of meters. There's no way a transmitter can detect a tag at kilometers of distance.

edit in reply to PeterJ's comment
RFID is the name of a technology where receiver tags actively load the RF signal (the carrier), and that load is detected by the transmitter. By pulsing the load a signal is sent to the transmitter.
There are lots of RF data transmitter-receivers which can reach up to tens of kilometers, but they're not RFID.

better solution
A better solution would be an ordinary transmitter equipped with GPS receiver, which broadcasts an SOS (I think there's an RF band reserved for this) including GPS coordinates.