Took the wheel over the LBS and had a discussion with one of the friendly people there. His opinion (without taking it apart) was that the race had become oval through wear and thus accounted for the play. Seemed reasonable to me. Bought a set of lock nut wrenches which I lacked and decided to just reassemble the wheel and live with it. After tightening the lock nuts with a reasonable degree of force the play was no longer there and the wheel turns nicely without any gravelly sounds. Good enough for me.