I've had an experiment in mind that I haven't tried yet. Since you can't have SPD on one side and toe clips on the other, I'm going to try attaching PowerGrips to one side of SPD pedals. Scraping the PowerGrips along the ground won't interfere with riding, and it probably won't damage the PowerGrips, or at least not much. That would be the ultimate pedal, no? Wear any shoes you want, and you'll always have foot retention. And if a guest rides your bike or if you just don't want to be strapped in, just step on the PowerGrips.