Yup, what @LuxXeon said is probably the logical way of doing it. Bake/project the high poly to a displacement, apply it to a lower poly mesh, and unroll that mesh.

Without seeing the mesh [how much relief there is in the carving/sculpting], I wouldn't discount pure brute force. Volume/area selection tools, and going in small increments, it'll take you a bit of time but its not difficult, just tedious [you don't have to be perfect, the process is usually somewhat forgiving]. In the days before modern UV unwrapping tools that was sometimes the only way to do it!

Or as @Lobo3433 mentions, you could just rig it. A ring of bones, rotate them until they're straight, and save the transformed mesh.

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