If you mean just certain layers, then you can flip horizontal by using what Joannesalfa proposed, with 180? as the angle. A vertical flip would be difficult (I can't think of any C2 option right now that would allow that, besides "set flipped"/"set mirrored" specifically for sprites)
If you are looking for a vertical flip, or if you meant a portion of the output (like just the left half of the screen) by "part of the screen", it would have to be done with an effect as of now. HLSL (used in my effect for CC) is pretty close to GLSL, so it shouldn't be a problem to use the "flip layout"-effect as a base for a C2 effect, for someone who already programmed a pixel shader in GLSL.
I'm afraid I won't have the time to do it, as I am concentrating on other things in my life.
EDIT: rotating the layer by 180? equals to both, vertical flip/horizontal mirroring at the same time.