May I vote for ellipsis overlaid on one corner of the element?

  -------...-
  | Primary |
  -----------

unobtrusive, "..." indicates more was omitted in every context I can think of, and it can be any number of colors because it's so visually distinct (err, of course, except for the same color as the background, but if someone was that evil, we're right back where we started from in the "I dunno, guess" realm)