I think as long as it's unglazed it should be fine. I think I read that the glaze can have something harmful in it. I think alot of frag plugs are ceramic and there are people who used to make ceramic rock to use as dead rock. I'm not sure how easy it is to find unglazed tile, though.
Thanks, I have some tile left over from the bathroom renovation and I was going to use one on top of the egg crate cage to level the skimmer. It's the perfect size, but I don't want to take the chance.