I recommend checking out the old Core Image Programming Guide on how to supply a ROI function.
Basically, you are given a rect in the target image (that your filter should produce) and are asked what part of the input image your filter needs to produce it. For small images, this is usually not relevant because Core Image processes the whole image in one go. For large images, however, CI applies tiling, i.e., processing slices of the image in sequence and stitching them together in the end. For this, the ROI is very important.
In your mirroring example, the first tile might be the left side of the image and the second tile the right side. When your ROI is asked what part of the input is needed to produce the left side of the result, you need to return the right side of the input image because it's mirrored along the x-axis, and vise versa.
So you basically have to apply the same x-mirroring trick you use when sampling to mirror the rect in your ROI callback.