The link you mentioned says
Note:
Adding a default value to a new metadata property (which doesn't exist already at the ./jcr:content/metadata node) does not display the property and its value on the asset's Properties page by default. To view the new property on the assets's Properties page, modify the corresponding schema form.
I believe you would need to add the new property to existing/new schema forms and apply that metadata profile to specific assets folder, it would certainly be available for the authors to view/edit.
Metadata Schemas