I have port 80 forwarded to a web server on our LAN. Occasionally we take down the server for maintenance, and I don't have another handy. Is there a way to have the 2760n serve some sort of page saying "The server is down for maintenance please try later" for example? If not natively, then by redirecting port 80 to one of the attached USB sticks?

i am not sure if will help but try at User Management > Advertisement > Advertisement ... to enable
& then
System Maintenance > Administrator Setting > Message ... to write the desired message

Not sure if will work but worth it to try

I got the impression that was for people actually trying to log into the router itself?

This is when someone log into the network will appear a pop up screen (new window browser) with the message ... i am using it and is working each time for anyone who is logging into my wifi network but not sure if will work with server

Note if they have blocker activated will stop the pop up window with the message

I can not think any other way if this not works ... sorry as i am not using servers