Yes, that glass-only warning put me off initally too, but it also says that using it on plastic is okay if the manufacturer says it is.
I have an HJC helmet and they say that Rain-X is okay. From the HJC web site:
Anti-Fog Inserts: After market anti-fog inserts such as Fog City and HJTech Anti-Fog can work very well with HJC flat (2D) shields. These inserts are not recommended for use with standard HJC 3 dimensional (3D) shields due to their double curvature shape which could cause a distortion in vision. Follow the installation instructions diligently.
Rain-X & Other Products: Applying RainX or other water dispersing type chemicals work well on HJC shields (except RST mirrored shields) if applied on new shields and if the chemical companies directions are followed diligently. Always use extremely soft and clean cloths for applications.
BTW: It works just fine on “old” shields’ (visors) too.
Check your helmet manufacturer’s web site FAQ or shoot them an email if Rain-X is not referenced.
Mine is a yellow 3.5 fluid oz. bottle called “Rain-X Original”.
I re-apply it after every rain-soaking; ‘ don’t know if I need to, but a small bottle goes a long way, so why not do it and be prepared.