SpeedTree shaders in Unreal are pretty good, and it does a good job of generating billboards for you with good normals. I suspect there's nothing revolutionary about what they're doing, they've just spent a lot of time and resource on getting them as good as possible.
Is anybody aware of any info about how things were made from the recently released unreal game Way of the Hunter, published by THQ? I'm specifically interested in foliage and billboard shaders most of all. I just took some look on artstation but can't find much. Perhaps somebody here knows something more?