This may be totally useless, but about twenty-five years ago a below-knee amputee rode past my wife and me. We just heard a click, click, click sound and away he went. He had fashioned some sort of prosthetic that had a hole at the end. He just slipped this over a bare pedal spindle. Getting "unclipped" would be a matter of sliding off the outside of the spindle. I'm not sure what was on the bottom of the prosthesis; he went by too quickly for me to see, but I suppose it wouldn't be too difficult to have something on the bottom that would give traction.