Board Thread:General Mod Discussion/@comment-29773086-20170504012214/@comment-25101089-20170504022340

That would require a lot more than a coremod.

Blocks have 16 states; slabs use these for (8 subtypes) * (2 placements). To have them orientable along all 3 axes would require (3 axes) * (2 placements) and then you could only fit in (2 subtypes), so slab blocks would need to be rewritten to expand all their subtypes out into 4 times as many blocks. Not only would this break compatibility with slabs in older worlds; while it could in theory be done using a coremod, it's along the same lines as theoretically rewriting the game to change the world height. Far, far too much code would need to be replaced, and with a coremod even replacing the smallest amount of code is a tedious, confusing, and fragile job.

So it's a no, and almost certainly a never.