I've added the code to produce the mirror image to "mirror" branch: https://github.com/genericsoma/fbtft/tree/mirror

It applies only to S6D1121 controler (fb_s6d1121.c). If should be easy to use the same trick with similar controllers such as ILI9325. Basically the starting address is calculated differently and the so-called GRAM direction is set to the opposite.