I didn't know anything about the Iridium satellite constellation so I checked http://en.wikipedia.org/wiki/Iridium_satellite_constellation to get an idea. The satellites orbit at an altitude of approx 483 miles.

It seems to my non-communications engineer brain that the communications would need to process the doppler out to get a stable comm signal. If true they should be able to use that doppler to pinpoint a location if the network software supported. If a signal was flagged as a SAR priority, Iridium may be able to use more than one satellite to obtain that geographic fix. I'd be surprised if the InReach folks hadn't considered that contingency. Something to ask InReach.