However, it is NOT available for use for subgrid ribbon customization. For example, if you add a custom button on entity new_a subgrid ribbon, this button will be shown when you click on new_a subgrid on related entity new_b form. But the referenced library for entity new_b form will not be available for new_a subgrid custom ribbon button.
Sucks! But makes sense given that the subgrid is available on all related entities form, and the referenced libraries on entity new_b form might not be added for entity new_c form and so on.
Cheers – Sy