I don't think they have, jonno. The reason, from what I understand, is that the curvature of the bridge is made to match the curvature of the fingerboard and raising or lowering one string relative to the height of the pickup would throw the relative volume of that string out of whack. I believe this is laid out in the FAQ section of this site in a far more articulate way than I'm capable of at 11 a.m. on a Sunday.