Lightroom 4 does not provide a built-in way to filter all photos that have been printed or exported. For prints, manual tracking is generally the practical solution. Your current keyword + Smart Collection workflow is a valid approach, and some users instead use color labels to mark exported or printed images for easier filtering later. For exports, Publish Services can be a better option if it fits your workflow, since they help track images sent to a destination. So the short answer is: no native global printed/exported filter in Lightroom 4; use keywords, color labels, or Publish Services depending on your needs.