Actually, I have things set up just like this. it might even work, but there is not really any way to know that the iMac is sleeping as opposed to having just shut off the screen, at least not that I have discovered.
It has been suggested that using the apple keyboard and using the CapsLock light tells you, but it doesn't in my experience. If the light is on and you sleep the machine, yes it does go out. However, it also goes out based on a timer, probably when the keyboard sleeps, so it is not a reliable indicator of actual cpu sleep.
In my present setup, I sleep the display and tell it not to sleep the cpu with the display. I believe this works. Then I set a schedule for sleeping the cpu overnight and waking it in the morning. I think this works, but who knows?
Terry