if you look at a machined rim, it's NOT just "sanded" or "filed" it has many tiny ridges and grooves like a ruffles potato chip.
once your pads wear in to the shape of those ridges, you effectively increase the braking surface.
I've looked into setting up a mill, with a small motor and a rubber drive wheel, and then mounting the laced wheel onto a fork, and then running the rotating wheel into a cutter,....but then I just look at how cheap it is just to go buy one machined velo deep V, and I always lose interest in the self CNC machined wheel.
the sandpaper trick isn't a half bad idea actually, especially if you use several finer grits in succession, and maybe even some felt, and some polishing compound for the final smoothing.
I think they'd be noisier than a cat with it's tail under a rocking chair tho