provide a quick mechanism to override the current schedule

I am envisioning something like exists on a typical programmable thermostat, which allow you to override the schedule without actually editing the schedule. I am using the signs in a church, and most of the time the schedule works well, however if there is a special service day or time change, I'd like to be able to say "play show X now" - either forever or until a certain time. After I cancel the override, or the time is reached, then the normal schedule resumes. Currently the only way I seem to be able to do this is to modify the schedule.