REM is a very refined polishing process. Nothing is deposited on the surface and nothing is changed on the base material. So any/all statement specifically stating coating, anodizing, painting, etc. does not apply to this process. The only premise to use for this process not being legal is that it falls under the unauthorized modification umbrella, except for R&P's.

Worth mentioning is the REM can make parts too smooth and that there is a diminishing return on the ultra polished surface finish. The example is some gearbox and R&P results from NASCAR that show REM gears fall back to a rougher surface finish after running the cars with REM processed gears. Get well maintained used parts and the friction surfaces will be polished as a result of regular use.

A side benefit could be oil shedding, but that is tough to quantify.