Sorry to say so, but I think you are out of luck..! This inconsistency appeared first in 10.2 and is probably due to attempts by Apple to speed up the GUI.
I agree that it would be awsome if it is was possible to have a fill that stretched between the two caps (as in carbon apps)... That would allow for some cool titlebars..

Here is desktopper.net's explenation
(translated with babel fish, the images and movies speak for themselves though)