you can edit default values of a pipeline asset with python like this
import unreal
eas = unreal.get_editor_subsystem(unreal.EditorAssetSubsystem)
pipeline = eas.load_asset("/game/interchange/MyDefaultAssetsPipeline")
mesh_pipeline = pipeline.get_editor_property("mesh_pipeline")
mesh_pipeline.set_editor_property("combine_static_meshes",True)
eas.save_loaded_asset(pipeline)