The <parent> tag only affects what other (non-syntax) extensions should be active when your syntax is applied. For a more detailed explanation of the relationship of syntax and tooling, see here. If injection à la Swift Stencil does not work for you, you basically have two options left:
- Fork the HTML syntax and add the necessary changes. This is likely a maintenance nightmare, as you need to redo this every time Nova updates the original syntax (though you might be able to partly automate this).
- Inject your scopes as Template Scopes, which has its own drawbacks.