Long focused on the needs of bike fitters and those with unusual physiologies, California pedal-peddlers Speedplay have until now had little to offer mountain bikers with uneven leg lengths. Because (after a point) shimming below an SPD-style cleat is impractical, Speedplay have split their Syzr pedal body, allowing the fitter to add 1mm shims to either side of the axle. While pedals were shown with one through seven shims installed, the application is limited only by the length of the fixing bolts (and ability of the rider to clip in on such a tall pedal).
Reviews of the Syzr have been mixed, but for those with special requirements the Syzr may well be the best option. Pricing and availability have not yet been set.