AppleScript cannot do this without additional software (unless you develop something with AppleScript Studio). I usually suggest and recommend Extra Suites for this type of request. It offers many features that are desired by scripters, including progress bars and dialogs (messages) with no buttons.
As a free alternative, you could script an application like text edit to open with a short message about the code processing, then close it at the end of processing. You could even show a dialog at the end of processing if the user could have started doing something else. Not quite UI goodness, bu a decent homegrown solution.