I read your first post a bit more in depth and well, it's a pain in the *** (a$$) making a res independent theme. I found this in the xlabel docs though:

"AutoWidthMode "left", "right", or "center"
Sets the horizontal auto adjusting method of the label.
"Left" keeps the x-coordinate given in (label-name)X constant (grows only right).
"Right" adjusts the the x-coordinate to keep the right coordinate (respective to initial (label-name)X and (label-name)Width) constant (grows only left).
"Center" adjusts the the x-coordinate to hold label centered to initial centerposition [(label-name)X+(label-name)Width/2] ((grows left and right identically)."

Maybe you could do something nice with it. =)