I’ve found an answer. For others that might face the same issue:
In the free Content Examples project, there is a ‘Math Hall’ level. There you can find some green planes that face camera using material math. The material that you’re looking for is named ‘M_CameraVector_to_Object’.
However, this doesn’t work with instanced meshes like painted foliage. To fix it, replace the ‘Actor Position’ node with ‘Object World Position’ node. Now it should work also with hierarchical instanced meshes, e.g. on painted trees.
EDIT: … This method messes up object’s scale though, so it’s not ideal…
… it has scalar parameters for object width & height, but I need to retain the original scale of billboards, not set it via parameters… It also makes the billboards appear upside-down.