In iOS 12 you can set an alternate face. You know have two total faces you can set for FaceID.

Amazing. I did not know this, thanks!

Huh. My iPhone X works perfectly with FaceID, since iOS11, clipped to my bicycle handlebars. I swipe up, glance down, and it recognizes me + bike helmet + polarized sport sunglasses + blue sky behind me. Seems like that's about as tough a use case as it could get short of a scarf covering my face (in which case I don't really think I want FaceID to work anyway).

Could be that your sunglasses block the faceID scanner. I have some brands of sunglasses that do not work with faceID, but other brands that work perfectly. I imagine it depends on the filter they apply to the lenses.

It works fine for me with safety goggle, bike helmet, etc.

Actually, you can. iOS 12 supports "alternate appearances" - you can find it in Face ID settings.

Do you really want anyone with a helmet to be able to unlock it?

iOS 12 now supports this. I don't know how well it will work with a helmet or ski goggles though, but it's worth a shot!