In training we practiced racking the slide on the edge of your duty belt or holster during a reload to simulate one arm being injured, but to do it as standard practice out of the holster seems like a trick at a shooting show as opposed to anything usefull in a real situation. And trying for speed while doing so is probably an invitation for an accident.
I've done one handed racking with my belt once or twice, I doubt that I'll ever need to do it any other way if one hand is not usable. That's why you should carry one in the chamber! then all you have to do to reload one handed is hit the slide release!