As mentioned something like Pam cooking spray with silicone in it would probably be the best release agent. But I think the bigger issues would be keeping the durability. If you are thinking of making rocks or something like that where the cement will be thicker then that would probably work. But if you're thinking of doing a BG or something where the cement would be thinner than a few inches then cement tends to fall apart in thinner thickness. Even a BG as thick as 1-2" would be hard to hold together on its own out of cement. Usually cement projects need to be pretty thick and even then need some sort of inner support to make it work. Just like with rebar in roads, no inner support and it just crumbles and cracks apart.