You could install these electric ones.....
http://www.badlanzhpe.com/index.html
.....and then get a cheap shift light circuit from ebay, hack it and use the ouput of that to drive a relay circuit (rather than lighting the shift light) and you could have a configurable exhaust valve.
If you could have the valves opening into bypass pipes then I think you could be on to a winner. I am not 100% on the sound but like the idea.