I don't know if there's a better way, but the way I came up with was to do the whole thing in Photoshop with semi-transparent buttons, then flatten all layers and save the whole as a jpeg. This was used as the background, and then completely transparent buttons were placed over the background jpg in the appropriate places. Don't know if there's a more elegant solution to this one. I ended up not using semi-transparent in my icf, so never went any further with my attempts.