You can enter isolation mode without releasing the clipping mask. Beyond that I don't see a way to do this automatically unless you want to make your stipples a compound path or group and use Pathfinder operations. Unfortunately AI doesn not have inside/ outside detection for these kinds of scenarios that wouzld allow to quickly select separate objects in this fashion...
Thanks for your really quick response Mylenium. I came back to the problem this morning with a clearer head and worked out that the Crop feature in Pathfinder did exactly what I wanted. Thanks for pointing me in that direction.