Enable (vs Disable) Glossary for this post?

Say I’d want to enable the Glossary for only a handful of posts on an existing site with many posts. It looks like I have to either:

• Toggle the Disable Glossary for this post switch in the meta box of existing posts, while leaving it enabled on the posts where I’d want the glossary feature.
• Create an entirely new post type and enable only that in the Settings for Post Types settings.

Neither is great for existing editors, who would either have to remember to switch the glossary off each time, or use a different post type for their posts – which again comes with some headaches.

Is there any way to “invert” the Glossary switch in the post meta box, so it defaults to “disabled”, and can be enabled when needed — or — a filter to enable the glossary only on selected taxonomies like a category or tag?

An experimental PHP snippet / filter, or any other approach to achieve the same results, would suffice. Ideas are greatly appreciated.