It would be cool to be able to mix slabs of different types on the same "block". I'm not sure of the implementation details, but this mod does it for Forge: https://github.com/CJMinecraft01/DoubleSlabs

If you think it makes sense to add this feature, I'd be willing to take a deeper look and probably draw a PR.

Closing as the project seems to be abandoned

Oh this would be nice.