I haven't tried this, but what might work as a very roundabout way of doing it is to export the photo into iMovie, and use iMovie's "Title" effects in it that let you add text.
Then you just have to figure out how to get it back. I think you would probably need to save it as a movie, then copy a "frame" out. Not sure what the resulting resolution might be, though.
FWIW, I think that I'd be more inclined to look around for a friend with a dSLR who already owns Photoshop, to see if his camera shipped with a copy of Photoshop Elements that he isn't using.
-hh