Have you tried BMX or downhill pedals with the traction pins? Maybe something similar to the Crank Brother's 50/50. I recently had a pair of similar pedals on the fixie because I had a cleat malfunction. With a pair of deep lugged shoes, I had quite a bit of traction. Much more than if I had regular cage pedals. It also might be cheaper than the Rivendell's, though it may not have the same cache.

-Rob.