Can I send a status message from an external (standalone) PHP script to the Craft engine?
Probably the easiest way to do this would be to add another segment to your URI.
i.e /bla/bla/bla/message-1
Then in your template, read the segment and display the appropriate message.
You could do it all in the template with a switch or conditional or the segment could be the entry slug for the message and you can pull from Craft entries.