I've always wanted to know how to do this-- basically seperate the flat base color of an image, and save it with transparency to .PNG. The only method I can think of is with selection tools and that gives a really bad result (not the same).
No, some of the modes can under some circumstances be applied to the images channel. But screen is
and can not be replicated without knowing the value of the ubderlying layer beforehand. Which would make using it irrelevant anyway.
The way you do this is by difference key where you chose one channel as alpha and then adjust white point to clear out the background.
Relevant posts for the removal of bg: