Ha, finally a thread where my recent experience is of some practical value...
I use the old fashioned stainless steel Maillon Rapides, which must be closed manually. I recently (Easter 2009) forgot to close one...
Riser left me on glide, after around ½h flying. Wing was still perfectly flyable as I was still hanging from the speed bar line. FULL bar on one side plus pretty hefty brake input, and normal on other side = flyable. I could JUST reach the riser to try and re-hook-in, but wing started collapsing every time I tried so in the end I deployed. Landed fine, bent 'biner back into shape with some rock aid, walked back up, relaunched, flew home. EXCELLENT adrenaline rush!
I suspect most speedbars would support ½ your weight at least until you deploy = redundancy (of a sort),
Mads S