Yeah, I've thought about that, but I'm going to do it a bit too much to do it pixel by pixel.
My main need is the first example: replace the alpha map. The rest are just "for extra credits", stuff I think I may someday need, but not at the moment.
Edit: For my needs, it would also work if I was able to add an alpha channel (from another bitmap) to an RGB bitmap, creating a ARGB bitmap.
Edit 2: It would also work to do the opposite. I could have a blank (A data only, RGB all black) ARGB, then copy RGB data from another bitmap, replacing the RGB data in the ARGB.
Just to clarify my needs:
I want to be able to colorscheme icons. I intend to do that by having a bitmap with the icon shape in the A channel, then make a nice colorschemed gradient on another bitmap. Then, somehow, I want to use the A channel bitmap as a cookie cutter to cut out the correct shape from the color bitmap.