It would be easy to implement an additional drawing function which allows flags to draw a given bitmap upside down and/or mirrored (which is a little bit more complicated as a lot of bits may have to be shifted around)...
Why it would be cool? You would need less images, especially for watchfaces (only 15 images instead of 60 for a hour hand) so some prg files and memory usage would shrink dramatically.
Why it will not be done? Because such features need new approaches in programming (e.g. compiler directives, like CompilerIf #SDKVersion>2 .... CompilerElse ...)