If the clock is on a wall, you can run the rod for the clock hands through the wall so they can be turned manually by a stage hand without being seen.

If you have access to the back of the clock behind the set, the arms of a clock can easily be attached to both a thin metal tube and a rod that fits snugly inside the tube. You can attach handles to the backstage side of the contraption, and manipulate the arms of the clock easily from behind, to your heart's desire.

The link is to a photo of such a clock which I made last summer for a play that was performed at a Fringe Festival here in Colorado. The clock was a portable cuckoo clock, and the cuckoo inside it was portrayed by an actor. It was all rather cheesy, but it fit with the style of the piece. The arms moved backward and forward at various times throughout the play, depending on the whims of the cuckoo.

http://www.brianmiller.biz/NotatHome4.jpg

Both the metal tube and the rod need to rotate freely relative to the case of the clock, with enough friction to hold the arms in place when they are not turned. I used a threaded steel bolt that just fit inside a threaded tube such as they use for overhead lights in your home. Both the tube and the bolt were secured in place with nuts at either end. With the right amount of grease inside the tube, they could be easily turned and then left at any position, and they wouldn't move until they needed to be moved again.

I likely can do a drawing for you if this doesn't make any sense. My description makes it sound a whole lot more complicated than it is.