Microsoft Equation Editor can construct fairly general equations, including
putting a bar over any character you wish. It comes on the Office CD, not as
a download.
I know of no add-in that would be relevant. I know of no font that would
have r(bar) on it, although you could create such a font with a font editor.
x(bar) is Alt-0214 in the Math Ext font from
http://www.columbia.edu/~em36/wpdos/wptoword.html
x(bar) is Alt-0120 in the MS Reference 1 font
Jerry