>>402500747
Looks about right, following your instructions ( alpha to red, blue pure white )
In photoshop, select layer -> layer mask -> from transparency. This extracts the alpha to a layer mask.
Ctrl-click mask to copy the selection to clipboard, then paste it into the Red channel in the Channels tab.
After that just paint the Blue channel white, and you get this.
Sorry no idea how to do any of that in Gimp. Probably write a python script to do it if you've need to do this a lot.