Hi @infoartist,
I think an alternative in your scenario could be setting Magento to display out-of-stock products, and then set the visibility for that product to be "visible in catalog", and uncheck all the categories for the product.
As a result, your product won't be included in search results, and it won't be accessible from the catalog since it hasn't been included in any category. It's good to mention that the product will be displayed in search results in Google.
By doing that, the product will be visible from the related products block, but it won't be accessible from the categories or search results.
Best regards.
Gabriel