I have the same need, assuming that I understand what you are looking for.
If indeed you want the frame to be alpha = 1, and the inside of the frame to be translucent (alpha < 1) then I think this is achievable for all frames (other windows would be similar in nature) through an appropriate looknfeel modification (which I believe can be done programattically).
See the following image for an example of a frame that is not transparent and yet the inside of the frame is:
and here you see that you even have control over the transparency of the frame and the titlebar (though its hard to see given its width that the red frame is actually alpha<1)
