Hi Chuck,
Your idea for an anti-backlash device is likely a good one. I would caution that too thin of a nut will wear out pretty quickly, because you've got too much unit pressure on the bearing sides of the threads. Most drag is not caused by the nut itself, but by how much pressure is required to move the slide. The nut is merely the fulcrum of the helix. The more thread you have, the lower the unit pressure, and the longer the nut and screw last without premature wear. Excessive length would create drag depending on the viscosity of the lubricant on the threads, that seems to me would be about the only source of drag. Of course, the threads should be smooth, too.