In Photos, how do I tag someone whose face is not visible?

On a Mac you can circle also the back of a head to name a person and add it to the People&Pets album.

While viewing the photo open the Info with ⌘I, then click the "+" in the section "Add face" and circle the head of the person. Type a name into text field below the circle.

This will add the photo to the album of the person, but the custom face will not be used to learn what the person looks like.

You can mark any part of a photo as a face, even toys or statues, anything you want to organize in the People&Pets album.

Have you tried to manually add the face:

BTW it is possible to export such face name with the original image as a .xmp sidecar, and then merge .xmp metadata that was edited inside Photos (faces, dates, locations, titles, captions, keywords) to the image's metadata with GraphicConverter or exiftool. Then Photos can import that face as a keyword as well as also other such updated metadata. Photos can't import .xmp sidecars and has many other issues <cough, bug, cough> with .xmp.

[XMP-dc]        Subject                         : Jamie

D'oh! ninja'd by léonie :-)

One person's tag is another person's... keywords. Add a keyword for Jamie...