I am the proud owner of a new (old style) Brunton 16DLU with a sighting mirror. I tried it out as a signal mirror. I faced the sun and got the reflection on my extended arm with my thumb pointed up and aimed at a close object. I was shocked at how far off the reflection is from where the reflection was relative to the top of my thumb and the target. Good horizontal alignment is possible but the reflection is going to be high from where your thumb and eye aligns with the target.

So if you have a regular mirror (no hole) and you need to signal a plane; you may not be seen even though you have a bright sun and good reflection. Your reflection will be way above the plane.