>>402500747
Looks about right, following your steps ( alpha to red, blue whited out )
In photoshop, select layer -> layer menu -> layer mask -> From transparency. This will move the alpha to a layer mask.
Ctrl-click the layer mask to copy it, and pate it into the Red channel in the Channels tab.
Paint the blue channel pure white.
Return to the Layers tab and delete the layer mask, and you should be done.
Sorry, no idea how to do it in gimp. If you need to do a lot of these write a python script with opencv/imagemagick I guess