I looked into this and ended up with a system like Norbie described. I even made up the plumbing in PVC pipe to see if it was doable. The hardest part would be getting a smooth transition from SC to turbo. You would have to run the SC until the turbos air needs overcame what the SC could provide...more a case of flow than boost. A bit of dyno time would sort it out but pretty fiddly. With the new generation turbos they have these days it's just not worth the effort....anyone want the SC12 i have in the shed?
Pete - as for the turbo providing positibve gains to the belt once it takes over?? It just doesn't produce the torque to have any worthwhile benefit.