We use these pens quite a bit at work, especially when trying to figure out difficult translations, a process that may call for a lot of revision. The pens are the same price as regular pens in Japanese stationers and come in a lot of colors. So I was curious when I saw that they were recommended by some for marking fabric.
I went to Pilot's Japanese website where there is a quite extensive history of their development of this "disappearing" ink and an explanation of how it works. <http://www.pilot.co.jp/frixion/info/index.html>
Basically, the colors of the ink are contained in microcapsules that are linked together in a configuration that makes them reflect certain wavelengths and thus makes then visible to the eye. When heated to a certain temperature (in this case, 65C or 149F), the bonds between the microcapsules change and the microcapsules no longer reflect visible light at the same frequency so the color seems to disappear. It is not actually gone but hidden. A return to color can be achieved by cooling the ink sufficiently (to -20C or 4F). Pilot calls it "metamorphic ink."
Nowhere in all the explanations on these pages did I find a recommendation for using this ink on fabric. Neither did I find an assurance that the ink would wash out. So, I would advise caution if using one on fabric. Perhaps some experimentation is advised before commitment.