Generator for creation of random links to give clients access to a project that is in maintenance mode

When I do projects for clients at specific points I want to give them access to the frontend to get some feedback. I want to use maintenance mode. I'd love to have an option where I can create random links that are excluded from maintenance mode that I can send to a client. It would also be cool if I could apply settings to those links (like f.e. an expiration date for that link). Also It would be cool if I could make some sites just password protected in maintenance mode, so that I could send a link and a password to a client. Both options would be nice and improve productivity a lot.