It does make perfect sense.
However, blending modes won't help in this case and not because Construct only supports a subset.
It seems that there are none that blend across channels. Alpha only blends with Alpha, R with R etc.
Yet I managed to find a solution - I'm in the process of cannibalizing the Color Mask Effect to do what I require - the logic flow is quite clear and easy to replace with a simple channel shifter - I have the basic operation done, but before releasing, I want to add the following:
-ability to manually select the channel to shift over to alpha
-a choice of either multiplication by the existing alpha or replacement
-an option to inverse the incoming channel
Once done, I'll post it in the Completed addons forum.
Thank you for pointing me in the right direction.