I quite often want to email diffs of files I don't have write access to, so
I visit the file, write it to /tmp, then edit it, save it, and go to /tmp to
run diff between the file and its backup.  But Emacs doesn't make backups of
files in /tmp, and there doesn't seem to be any way of asking it to via the
customize interface.

The best I've found is to:
  (setq backup-enable-predicate '(lambda (file) t))



Suggestions:
1) make that the default value - why not write backups in /tmp?
2) offer a way for the user to customize the behavior without resorting to
writing Emacs LISP code