Here's a possible answer LancerMR99. Wheels magazine November shows muffler used on their latest super dooper 6.2 ltr twin turbo commodore.
Not suggesting you use that muffler! It might be possible to get one the right size made.
Muffler which is divided into 3 chambers, has single entry straight through pipe on one side with holes drilled into it and there's a flap fitted to this pipe at the rear of the muffler . Opening the flap electronically, gives you a straight through exhaust. Closing the flap allows the exhaust flow to escape into the front chamber of the muffler through the holes in the straight through pipe, then through a series of baffles through to the second outlet on the other side and gives you a muffled and normal!! exhaust.
Could be interested in one of these myself.
edit. Note XForce did make these and possibly still do.