What I do for foliage is set the face normals to a different direction than what’s automatically calculated, i.e. for a grass mesh, facing directly up. There might be a way to generate spherical normals for the bunches of leaves, but since most applications expect normals to be adjusted every time you make a change to the model, I’m not sure of the exact workflow to make sure the tree trunk doesn’t get affected and you get the right mesh for import.
If you can fix it in the application, then the engine won’t need to do so much work generating normals frame-by-frame: it will just work the way you expect.
EDIT - Sorry, rereading your post it seems you actually have done this and it’s messing with fresnel to cut out meshes at grazing angles. If there is a way to bake normals into vertex colors the way Deathrey described, that would be amazing! I know UE4 accepts multiple UV channels so you could bake to a texture, but it only accepts one set of vertex normals and one set of vertex colors.