You should be able to rebuy before the tournament starts. You'll have to have rebuys enabled (of course). When you attempt to rebuy, you should get a message stating rebuys aren't allowed because the tournament has not yet started, but you should be given the option to override this.
You'll also need to have the "Allow players still in the tournament to rebuy" setting enabled as well, because before the tournament starts no one will have busted out. In 2.6.b5, that doesn't even have to be enabled, because on the Rebuy dialog you can uncheck "Hide ineligible players" and rebuy anyone at any time.