Hey, Lorne.
I never found a perfect solution to my problem. In my case, I had to recreate a lot of the photoshop work in Nuke. Your case sounds different.
I’m not a regular Photoshop user anymore, but is there a “preserve transparency” button that you can select when you save a .png file, like when you save a .tiff file? If you had the transparency baked into your file, I’m not sure if you could then use an overlay blend mode to get the desired result. I can’t off the top of my head recall how the pixel blending math works for an overlay. I must admit, I don’t really follow your question 100%. The reality is that blend modes work differently in different programs, even though they should in theory work the same in every one. So, I wouldn’t be prepared to have the same results as in Photoshop.
Film Editor, London UK
http://www.scottclementseditor.com