I know there was a pony hair company that sold or sells scents you could boil the plastic hair in, but I don’t remember which one it was!
Best of luck in getting better answers. I’ll be watching this one! My childhood Sugar Apple still has a bit of her scent and I frequently lock her in a ziplock bag to amp it up. But if there’s more I can do to bump up that nostalgic scent I’d love to know!