I see - colour to alpha on the fuzzily selected area. Exactly. In truth, that I often do is duplicate the layer, fuzzy select the BG colour of the bottom layer, grow the selection a few pixels (especially if there is a drop shadow/omni shadow) then clear, on the top layer duplicate do a colour to alpha on the whole image using the BG colour, then merge the two layers. A little script might be useful with parameters for the fuzzy selection amount, the pixel grow size, and a colour picker for the BG.

-Rob A>