how to save a face in Lisp?

[Drew Adams]

In Lisp, how can I save a face to the user's custom-file after it has been
changed (or to cause it to be saved when Emacs is quit)?

For a variable, it's enough to call `customize-save-variable', but I don't
know the equivalent for faces. Perhaps it would be enough to `put' the spec
as property `saved-face' for the face? Or as property `defface-spec'? I'm
not sure how this stuff works.

[Obviously, this code would only be invoked by the user, aware that s?he was
saving the changed face. I would not save settings behind the user's back.]