There's a product called the DB Dawg. No experience with this:
http://www.dirtwerkz.com/mm5/merchan...egory_Code=260
Basically it's an insert you shove into the end of your pipe. It's held in with set screws, so you can take it out pretty easily. You'll need the one suitable for your pipe inner diameter.
I might have to buy one of these things... I picked up a track bike at the end of last season and it has an aftermarket exhaust. Not truly obnoxious, but the noise level might be marginal for one of the tracks I ride.