A good lap needs to be a perfect mate for the surface it is supposed to correct. You would probably have more error in your home made nut, than exists in the ballscrew to start with
You'd need to cast the lap "in place" in order to get a good replica. The lap should also be quite long, in order that it can average out surface errors by many points of contact.
Circulation of the abrasive is also important: a lap is a cutting tool and needs to have a place for the swarf to go. Otherwise, it plugs up in about 10 seconds
Ideally, the lap should run right off both ends of the screw, to insure that you don't end up with a loose fit in the middle, and tight at the ends.
Also, if you can reverse the lap (I'm imagining it in half-shell shape), you will improve the symmetry of the groove.
There is no guarantee that you won't spoil the shape of the groove somewhat. In the first moments of use, a lap becomes impregnated with abrasive (because its soft) and it ceases to wear as quickly as the screw. Grinding wheels have the same property: they are bonded together with a relatively soft matrix, and its the particles that make them seem hard.
If you cast the lap, you should use some kind of a "disposable plastic film" to space it away from the screw by a few thousandths. This is because the abrasive mixture should be of uniform thickness, and the plastic film would help allow for this to occur. In reality, you want the lap to be offset slightly from the ball track: if the ball track has a.0625" radius, then, you would allow .002 for abrasive, and want the lap to have .0605" radius. This will help ensure even wear of the screw. Any high pressure points will be the first to go, and if your lap does not fit properly, you'll just be ruining the shape of the groove.
Perhaps it would be possible to rig up a grease gun (or something) to inject the unhardened casting material into the mold, forcing the plastic film tightly against the screw while the lap hardens. Then, peel the plastic film off and start lapping.