Disclaimer: please read post #3 too for clarification
SpawnHeightOffset do work in CustomObjects, but don't seem to do so in CustomStructures.
The BO3 file is here a 8x8x1 glass rectangle with settings as written in the code block at the bottom of this post.
CustomObject it generates according to the settings and covers a lot of the surface area.
The exact same BO3 file
gr1.bo3 with branch
WeightedBranch(0,0,10,gr1,NORTH,100) generates like this as CustomStructure:
Images say enough. The described settings don't work when the object has branches.
ExtrudeThroughBlocks: DIRT, GRASS