"good signaling mirror without a sighting hole" is somewhat self-contradictory.

I would wonder if the mirror on a compass would work for your purposes. There is a hole, but it is on the side of the mirror. Most compass mirrors are glass, but they are typically fastened securely inside the top cover of the compass and are very unlikely to break. I have carried compasses of this type for decades and have never broken one, or even heard on one breaking, for that matter. I would think that if you did break a compass mirror, you have been involved in major trauma and would be dealing with very significant problems.