let's say you wanted to peg the bitcoin to the dollar. you just place huge bid offers at $0.99 and huge sell offers at $1.01
*note* you may want to use other people's money to do this, since it's pretty dumb.
You could do this on MtGox. Be sure to use dark pool bids, so that you can bluff how big your orders are: another thing that goverments and central banks are good at. Bluff that everything is OK right until the moment everything goes down the drain in 5 minutes. Then still call it a succesful policy and blame other people for breaking it.