Good question and I don't know the real answer.
I have had a measrure of success dressing a diamond (or also CBN) wheel with just a soft mild steel rod for a dresser. The trick seems to be that you cannot run the wheel at full rpm. For me, this meant just leaving the wheel mounted, starting the grinder up, then disconnecting the power and pressing the steel against it (and pressing quite firmly) and bringing the wheel to a fairly quick stop while traversing the face somewhat. All you can hope to do is attempt to crush the bond of the wheel and make a few of the dull diamonds drop out.
Perhaps you could rig up some kind of roll crusher to do this job if you have a really mishapen wheel, although the diamond layer is not likely to be all that thick.