Well technically leg locking will allow you to skid the wheel which complies with the letter of the law. In practice, I suspect a judge will want evidence of something recognizable as a brake. And yes, the skidding provision implies a rear brake and is absolutely stupid as anyone can make just about any wheel skid and has nothing to do with stopping distance. Someone correct me if I'm wrong, but I believe this comes from the Uniform Vehicle Code

In one of the rare instances of Mass refusing to adopt UVC, we have a much more useful requirement: "Every bicycle operated upon a way shall be equipped with a braking system to enable the operator to bring the bicycle traveling at a speed of fifteen miles per hour to a smooth, safe stop within thirty feet on a dry, clean, hard, level surface."

Fascinating! A requirement that actually bears some relation to the performance of the braking system in question! Of course it's still generally believed here that not having a separate braking mechanism would leave you open to charges of no brake.