Does anyone know how to legally reserve space for a truck in a metered zone in downtown Zurich?

Ask at your local police station and they will do it and charge...

All you need to know about parking and special parking permits: link to Stadt Zurich website, in German language.

If I remember well, you can buy the permits on line. Then you print them and stick to your car (normally, a police agent puts signs and warning in the parking spot you have reserved).