I think the difference between Ridley's shadow and metal face's shadow, is that metal face was shown in a cinematic cutscene. It would have taken a lot more to create a full model for the character than Ridley because it would be special for the trailer, while Ridley's model was already made because it was the shadow of an in game model. It would have been a waste of time to make such a complicated model just for two seconds of a trailer for a different character. I think that's why they only showed metal face's shadow. Ridley's model was already made, however, so that seems more like they're just hiding something.
edit: greninja'd