the parts to do this using an actuator can be bought in a kit already. i always think i've come up with a good idea only to find it's been done before

http://www.atpturbo.com/root/releases/release091004.htm

mind you, at $235 i think i'll make my own using a couple of old throttle bodies. and i don't like the look of the tee-piece that they're using - doesn't look the best for flow. it would be better to have like a Y-piece and the flap operates in the middle somehow and closes off the normal silenced exhaust route and provides a smooth exit straight out.