Goldfinch, generally Speedplays do not release when you are in a crash... at least, that has been my experience. But, I still would not use any other pedal.
I just unclip lying there in road, on my side, no having to take the shoe off. I guess if you're cognizant enough to do that, it's fine, if you're not, you have worse problems. But, with dogs nipping at me, I can see why you are freaked out.
I had a dog issue in my neighborhood a couple of years ago. When I saw I wasn't going to be able to outrun the dog, I immediately unclipped, before I even stopped and just rested my foot on the pedal. I mean, I unclipped right away, on one side, before I made my decision as to what to do. As you did, I decided to stop and put my bike between myself and the dog and scream my head off. And, to top it off, I was at the beginning of a little hill. The owner (my neighbor) heard me screaming and called the dog away.
I am sorry this happened, but this also happened to me with spd pedals (not releasing in a crash), in a busy intersection, so I would chalk it up to something that happens, not necessarily related to Speedplays.