Having it set up to work automatically would be awesome, although the options you said would work as well. I raid lead, so often watching the clock can be a bit hectic. How difficult would this be to code?
I don't think it belongs in NRT. You could make a separate addon that does it easily.
First you could hook NRT:TakeAttendance and use AceEvent-2.0's ScheduleEvent to schedule a event 60 minutes from then and fire another TakeAttendance. Then when you leave the raid just cancel the next scheduled event.
The whole addon would probably be 10 lines of code.
I'm not sure why you'd want to take attendance at exact times anyways. When we switched to an attendance-based DKP system we realized very early on that staggering the hourly calls would be needed to keep people from going completely afk except for when they knew the calls would occur.