The hearth must be composed of noncombustible material. There is no wiggle room around that. If a manufacturer of a synthetic brick you are interested in cannot claim IN WRITING that the product is noncombustible and suitable for the purpose you intend, I would not use it, because you incur a safety risk, which may be compounded by building code violation risk.

Your mantle can be made of wood so long as you observe the required clearances, the distance between the edge of the firebox and the edge of the mantle, which must be bridged by noncombustible material. Prefab units typically specify the clearances in their manuals. If you're planning a custom-built unit, you need to rely on the local building codes and the expertise of your installer.

You might want to post your question in hearth.com for further comment.