Very interesting! I do not think this is only placebo; it might actually work. When in a lucid dream, conscious and subconscious parts of your mind are in closer contact than in any other state (apart from meditation of course). Therefore I think that any kind of mantra/affirmation/autosuggestion is much more effective when in a lucid dream. You can 'speak to your subconscious' much more directly, causing the message to come across more clearly and more effectively.

Of course this is no proven fact and please correct me if I'm wrong in making these assumptions