no, I have not.
the bars require the 3 bolts to be tight, as the strength of them in tension reduces the shear forces on those puny bolts.
placing anything there reduces the hard face to face contact of the riser to the tripple tree, thus putting the bolts into an undesirable shear mode.
don't do it, there will be no way to keep the bolts tight, and the results may be dangerous.
besides, placing rubber there just allows the bars to move more, at a different harmonic.