I've noticed there's a new(new to me at least) status bar method SetReverseFill(1nil), which would supposedly fill the texture from the other side, but here is what it does(2nd and 4th row):
(i used your code just with bar:SetReverseFill(1))
So you still have to use SetTexCoord for R2L and T2B orientations, unless u want that wierd sliding behaviour.