I think I see your point (correct me if I don't though), and I think the difference between this technique and generally trying to incubate a dream containing dreamsigns is the fact that you do the incubation whilst being in a dream. I feel like you are more closely and more directly connected to your subconscious in a dream, opening up new possibilities for interactions. I don't really expect it to work after one time, but if you can manage to plant the suggestion firmly into your subconscious it might just work.