Establish a variable to act as a flag for emergency closure.
Send incoming calls during the day time profile to a module that checks the value of the variable, and if it is not set to emergency, sends the call to the desk. If the emergency variable is set, it sends it to a pre-announcement stating that they are closed, and then to the night auto attendant
At night, when the time profile sends the calls directly to the night auto attendant, also check the variable and play the pre-announcement if necessary.
Add a hidden option to the menu for the Night Attendant, to a module that will announce the current value of the emergency variable, and let you change it.
Finally, think about what covers the desk in the day mode, is it a Day auto attendant, or a voicemail? Have a path from the desk's coverage point to the module to toggle the emergency closure variable.