As I said WED has no problem handling concave blocks. It displays and renders them just fine. But you have to turn it on in map properties first, otherwise it won't work. It's the map compiler who is messing up things.
I thought this worked before, but I might remember wrong.
And if WED can handle meshes, why not consider blocks as meshes?

Just because WED used to work with concave blocks only, why still come with that argument? CSG with concave blocks is the past!
Blocks basically are nothing else than predefined meshes. WED should handle everything the same! So should the engine.