FTP doesn't work like that. Don't let the abstraction done by browser makers fool you into thinking it's ANYTHING like HTTP and loading web pages. They're two completely different protocols.
Generally speaking, without knowing which FTP program we're talking about though, it's impossible to give an answer, and by and large this isn't part of the FTP RFC, so anything beyond the RFC will likely limit the number of clients it works with.
Depending on how people login to this FTP server, you might be able to do something like Mark suggested. Assuming people come in via a web page link, you can just remove the link and say that the server is down for maintenance, then put the link back when done. If people are using an actual FTP client, then there's really nothing you can do. If the server is down, it's not going to accept incoming requests. When websites put up a "down for maintenance" page, they do so by loading it onto a secondary webserver. There's basically only an on and offline mode for servers, and if they're offline, they're not accepting any incoming connections, even to deliver some kind of "Down for maintenance" message.