I tried hacking up my idea for implementing buffer-local faces, here's
a patch.

The user interface is the variable `face-remappings':

   Alist of face mappings.
   Each element is of the form:
      (FACE . NEW-FACE)
   or
      (FACE NEW-FACE MERGE-FACE...),
   which causes NEW-FACE to be used where FACE normally would.
   If present, MERGE-FACE... are merged during display with NEW-FACE.

E.g., try evaluating the following in some buffer:

  (set (make-local-variable 'face-remappings) '((default . bold)))

It works (or seems to!) with other special faces like mode-line, etc., too.

Here's the patch; lemme know what you think: