Alpha Transparency in Skins...
I know this probably has been suggested a bajillion times... and the answer I've usually heard was, ~It would be easy, but we're still thinking of ways to implement that concept in the GUI.~
So... how about a little checkbox to set the final baked skin alpha transparency to that of the tattoo's alpha transparency? Just add a little toggle between applying the tattoo's alpha channel before baking (default) and after backing?