Re: luminance channel in AE

found my solution - put an adjustment layer over my video & dump my desired effect/s into it. Put a solid above all this and apply some form of greyscale masking image (eg ramp, fractal noise, a feathered mask, whatever, possibly then tweaked via levels/curves effect). Then simply switch the adjustment layer's transfer mode to "Luma Inverse matte" - it works just like a luminance mask in photoshop, selectively filtering the passage of filters/adjustments to an underlying layer. I can even alter the adjustment layer's opacity to fine tune the intensity of the effects' application, etc. This is exactly what I was seeking. woohoo