Not to re-ignite an old topic, but I built a lanyard with a 3/4" o-ring as the breakaway. It took about 15 to 20 lbs to snap the o-ring by itself. So, figure aout 30-50 lbs of force on the lanyard before it breaks free. While it won't rip your head off, I wouldn't place money on getting away injury free.

The breakaway buckle I scavenged from a Home Depot lanyard snapped at about a third of the force. I adapted it to a cord lanyard using d-rings, but in retrospect I also could have used a prusik knot.