No update on this. This is the way subtasks were designed. We know for a fact that Asana is slowly adding features around subtasks to make them easier to use for those use cases.
In your case you can look at F2 and F3 🔥 Hottest feature requests and their workarounds Your best chance is to add the subtasks as tasks within the project by multi-homing them. This can be done with au automation (custom made).