I'm a bit confused. Did the body release from the Blackrapid or did it release from the lens? It sounded like you had the FastenR connected to the tripod mount of the lens. Or did both the camera and lens still mounted fall off the Blackrapid?
I use a Yeti Blackrapid. I love that thing because it's convertible from a one camera sling to a two camera sling. I haven't had a problem yet. On occasion, I might forget to screw the carbiner lock. Maybe that's what you forgot, too?
By the way, the fact that Blackrapid sells a tether doesn't mean they acknowledge a fault in their design. They could also be acknowledge human error and have provided a failsafe. I might consider it myself.
Must have happened around the same time wife and I drove by UMass Boston to see what the snowfall coverage was like. Did you get to take any photos before the incident?