i think if you managed to find an old barbie doll, unless it was in horrible condition, it would be worth too much to do something like this.
Even then, i think some of them had anti microbial stuff, well ive never seen a barbie doll for example grow mold on it. Or it has some anti mold type stuff, and i would not trust the plastic those days.
Unless your talking about a Victorian bisque doll, which then oh boy... those dolls are things i get nightmares from, and having one in my fish tank staring at me covered in coral is a whole other level of a horror in your tank. Especially since the current / urchin / something can move it, and freak you out even more because it moved.