I don’t see any major benefit to what you are proposing. Trading on an exchange is a matter of updating database values. Displaying BTC or tBTC is a matter of updating the UI.
Testnet coins are intended to allow you to test your program’s ability to do things such as sign/broadcast transactions, detect incoming transactions, use coin control, and other wallet functions that occur on chain.
True, now that I think of it, I'm going to put this idea on the back burner for a bit as I don't want to "play roulette with the law" and like BrewMaster said how there are exchanges already doing this (with fake balances).