Hi,
I second this feature request and think it also would be good, if transformation tools would be aware if a layer is just a "regular" layer, or a layer containing a normal map.
As soon as you start to rotate a normal map, the stored vector information is not correct anymore. Flipping along an axis can be done, when swapping the rgb channels accordingly afterwards. But rotating values different then 90° or 180° needs to be calculated and there are actually just a few specialized tools out there that do this job.
So painting, creating and being able to rotate normal maps - that would be an awesome feature!