^ Agree with mudboy on the top lug only. It would definitely be the cheaper solution. However, you're not likely to gain more than one or two thread revolutions this way. Is the steerer tube steel? If so, you can have a frame builder either replace the steerer tube or braze on an addition, and then rethread the whole thing. However, this will cost significantly more.

Alternatively (and this might just be crazy talk), I wonder whether using smaller loose balls on the upper race would gain you one additional thread by reducing the adjustable cup height, without affecting headset function?