There's a difference between unpublished and unlisted. You can publish an add-on as unlisted, which will prevent your add-on from appearing in the G Suite Marketplace. You could do that. That's really the only option.
If you used a library, you'd still need a project file bound to the document. Of course, you could copy a template file which would automatically keep the bound script, and I'm not sure whether the library setting stays attached or not. I'm guessing, no.
So, your best option is to publish unverified and unlisted. Technically that's still considered published, and you can see it in the G Suitemarketplace IF
you know the link to get to the listing. So, it's in the Marketplace, but unlisted. It won't show up in a search, or just viewing add-ons.