Or just give me a better working distance?
The net effect of close-up lenses (diopters) is a shortening of focal length. I do not believe that will help you much with a LF camera, where obtaining close focus is usually not that much of an issue as long as the rails are long enough.
I also believe that a diopter will potentially cause field curvature and maybe other optical problems too. That may not be much of an issue if you frame a portrait in the centre of the image, but will be progressively more visible to the edges and corners. I have not tried diopters on LF before, but I have used Nikon's 5T and 6T on my 645 Mamiya system with 150, 210 and 300 mm lenses. These are high quality diopters, and the central image quality is very good, but the edges suffer even on a 6x4.5 frame, mostly due to field curvature. One has to stop well down to get acceptable overall sharpness. So my advice is to test with someone else's diopters first, before you invest money in an uncertain outcome.
As an aside: It might also give you a very interesting effect, perhaps similar to a Holga? Would love to see the results if and when you try. Recently I bought a jumble of photographic bits and pieces that was 80 % junk. I was after two bulk film loaders in the lot. One of the surprises in the box was an Arreiflex 95 mm filter/diopter. I have only one lens with a 95 mm thread, the Nikkor 50-300/4.5 ED, so I decided to put it on and see what transpires. It turns out to be a close-up diopter that reduces the min focus distance on that lens from a modest 2.2 m to more like 0.7 m. I did not notice any image quality deterioration up to about 200 mm focal length, but beyond that and especially at 300 mm it creates a strong halo towards the edges of the frame. The effect is removed by stopping down, and is no longer visible at f8, where the image quality is again top notch. Strange, and rather useful for tight portraits shooting wide open.
