The easiest way to restrict privileges is to turn off withdraws on your mtgox API key if you are concerned about this.   If you still want to register and are concerned about temporarily enabling withdraws to send the registration fee, you can send all but 0.1 BTC to a bitcoin wallet temporarily, and then register, and then send your funds back to your mtgox account after the registration completes to ensure that extra funds cannot be sent.  Other than that, I can only offer you my personal assurance that this software will not "empty your mtgox account" as you put it.