Not directly answering your conundrum on wrist computers, but—I go trail running frequently (in Hong Kong), so I've thought a bit about wearable devices and safety. Here are some of my solutions and thoughts:

• I use a forearm armband (example) to hold my phone in a position that allows me to use and see the touchscreen while running. I find this incredibly useful for checking a GPS map on the run while keeping both hands free for falls. I worry that the current generation of watches are nowhere near as capable as my phone.

• I rely a lot on Strava's online route creation tool and phone app for navigation.

• Digital personal locator beacons on the 406 MHz channel (example) are the current gold standard for distress signals.

• Sharing your location through your phone (e.g., on Google+) can give some peace of mind to your family and friends.

• An inactivity detector based on a phone's accelerometer might be a useful dead man switch for sending a distress SMS/email in the event of an accident that renders you incompetent. I haven't gotten around to setting this up on my phone, but here's an (untested) example of an app that might work.

• In case of emergency, it might be useful to have a GPS app on your phone that can display your grid reference so that you can tell rescuers where to find you.