I don't think there's any rear projection in the movie, other than in the small number of scenes of people in cars.  I don't know how you could even rig a rear-screen for the shot of Chaney and the gators.  As far as I know, the only shot in the lab scenes that involved optical effects was the scene of the ray projector in operation; the ray is an effect, but the machine isn't.  I'm sure that all that stuff had been constructed in the prop department, though some or even all of it may have been built for another, more expensive, movie.  Most of it was probably reused in other Fox productions.