Another idea relating to induction design (if you have the room): operate the supercharger and turbo completely independently of each other. Run a blow-off valve on the supercharger side. On the intake plenum, mount a throttle body on each end, with two stroke style reed valve after the throttle bodies on both ends. On the turbo end have a blow off valve set up to work in reverse. How it could work is at low rpm, the supercharger will make more boost than the turbo, so on the supercharger side the blow off valve is closed, and the air is blowing into the plenum on the supercharger side. The turbo side has the blow off valve open, allowing it to build boost. The boost pressure is trapped in the plenum by the reed valve on the turbo end.
At high rpm (or whenever turbo boost > supercharger boost) the supercharger blow off valve opens venting boost, the turbo blow off closes, pressurising the turbo induction side allowing boost to enter on turbo side of plenum, the reed valve on supercharger side closes trapping boost.
The problems would be having to rig up cables for two throttle bodies, room and flow restrictions caused by the reed valves.
You could do the same thing with a "2 into 1" pipe with an electronically controlled vane valve, and single throttle body. Actually, maybe a vane valve, lightly sprung to be in the middle (half open half closed on both sides) would do the trick.