\hrule\@height.8\p@ % adjust for the desired rule thickness
\mathpalette we make a macro that will do the right thing in all sizes. The first
\sbox command sets the argument in the desired size (display, text, script or scriptscript); the second one sets the argument overlined; then we measure the difference, thus computing the clearance between the symbol and the bar above it. Then we draw a rule with the desired thickness, leave the computed clearance and print the symbol.
It's not exactly as wide as
\bar, but it should be close enough.