Based on instructions from the maritime chapter of Club DSM (authored by Warren Tsai), I now have a soft blue LED giving me a visual warning of engine knock, a situation where gasoline is being ignited by compression rather than ignition from the spark plug (which can be caused by excessive boost). Total cost of this modification: about $5.00 worth of components from Radio Shack and an afternoon of rewiring my a-pillar to accomodate it (I finally broke down and rewired the gauges using clips instead of crimping the wires together, making it easy for me to pull the gauges individually without removing the a-pillar entirely). So far, it behaves exactly as Warren described, so I'm calling this a success.