Most systems have a way to add a button (with lamping) to put the system in "Night mode" basically setting the Override.
Pressing the key once would set the schedule to All Hours Closed and lite the button, Press it again to remove the override.
I can see this happening in two different ways.
1) Add a star code and an extension # for the schedule so *721 + EXT + 1 for day, 2 for night, 0 for no override
2) Add a button option "Schedule" in the Extension field have a pull down of all the schedules. One press sets Night override and the LED turns red, press it again and it removes the override and turns the LED off