I want to like this theory, but I'm not sure that the nonsense words sufficiently obscure the real prophecy. For example, if Voldemort tracked down the student, somehow reversed her Obliviation (if necessary), and then used Legilimency to find out the wording (if it turns out that Voldemort doesn't know this part?) then I'm pretty sure Voldemort would be able to reconstruct the original words.

For that matter, we are also told that "only someone who heard the prophecy in the seer's original voice would hear all the meaning that was in the riddle." Therefore what Voldemort would actually be interested in, given access to all of Rianne's memories, is Snape's own interpretation of the prophecy, or even any hints at it.