Let's look at Craig's question from a motorcycle-operator's viewpoint. That is, he did say he "...wants to ride a sidecar...". He might mean he wants to be the person at the controls -- after all, we ride bikes, we drive cars.
If this is the case, then instead of finding him a venue where a monkey is allowed -- someone needs to decide if a bilateral above knee amputee will be allowed to participate, perhaps by examining the rig to see if it has appropriate controls and modifications to let him be safe in case of incident -- and also to see if it still falls within the rules of his chosen class.