https://www.youtube.com/watch?v=1EnDXqOGUpM - skip past the short interview and then there's a bunch of be/age regression related FX tests.
This is a funny video from a studio testing prototypes for the Death Becomes Her BE shot animatronics. I'm sure if you look through FX studio youtube channels, you could uncover a lot of similar material with behind-the-scenes looks at your favorite BE scenes. I never thought of it before, but I'm definitely going to keep my eyes out for more.