Is there a way to buypass maintenance mode?

Hi, when creating site you often need to show it some 3rd party (e.g. payment gateway provider) before launch. I can imagine following two solutions: either disable maintenance and hope they will take a look fast enough or give them admin account so they can access site while in maintenance mode.
Both solutions are far from being ideal.
Can you please recommend some better way to bypass maintenance mode, e.g. using some query string in link?