There is some information about lapping leadscrews in "Foundations of Mechanical Accuracy" by Wayne R. Moore of Moore Special Tool Co. What Thrud refers to. But they do the lapping in a controlled way to achieve particular dimensions.
I think Oso's right -- simply lapping screw + nut together isn't likely to work very well, if you're after a high precision fit.
What I've done to polish a thread is get a soft pine stick, dip the end in some fine lapping compound, and press that against the revolving thread. The wood quickly conforms to the thread form (assuming a relatively fine thread pitch) and holds the lapping compound quite well. I wouldn't necessarily call this technique "high precision" either, though.
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