I’ve added a feature request (UE-41902), but I’m not sure when or if we’ll get to it.
One complication is what to do when packaging for both ES2 and ES3.1. The source landscape texture data needs to be transformed and packed when packaging for ES2, but if ES3.1 doesn’t require it then we would need to support both packed and unpacked mode simultaneously. I guess we could always pack when less than 4 layers and don’t pack when 4 or more layers.