I agree that it's a risk, but they haven't done it so far. Even if they did, Safebox might still be useful for people using other cloud storage providers (or even corporate file shares).
Well, if you did not quit your job to do this project and did not spend all your savings on it - the risk might be worth it. Good luck! :)
Is it possible for Dropbox to update in any way that could let them read it? I'm not sure what you're worried about.
The answer to your question is: no, it's not possible. I was rather thinking about dropbox implementing the same feature directly to dropbox. If they do, there would be no point in using denishennessy's product.