You should be able to do the layer combine in Images Plus. You might have to split the RGB image first into it's component channels.
Alternatively, using PS, don't insert it into a new channel, simply overlay it as a layer, select difference as the blending option, use your arrow keys to fine tune the movement (if you upscale first by a factor of 3 or 4x and then downscale to original size you will get finer adjustments for you movements) until it is aligned and then change the difference blending option to luminence. Do any adjusting to the layers to get them to look nice (you know sharpening, etc), flatten and voila!! Instant Luminence layered RGB