Does anyone know of an inedible alternative to fondant for a faux cake?

There's a thing called cold porcelain that might do the trick. I'm sure you can find a recipe for it if you Google it.

Why don't you want to use fondant?

thanks for the tip.. i'll look it up.

it is a cake for a theater and they want to keep it, but they are concerned the fondant will attract critters in storage...

Almost anything will attract critters in storage. The will even get into styrofoam. They will have to store it in a plastic container.

right, but i guess a non-food product is less likely to attract than a sugary-sweet smelling cake.

I don't think it will look like fondant, but you can use spackle from the hardware store. It's very easy to apply like icing, and very durable (yet lightweight).

Lightweight plaster works beautifully. You can even thin it a bit and pipe it too.