Based on what I've been able to determine, this is no longer functional (since around Washington). Previously you could upload JS files using the Upload.do UI Macro and they would be listed below the Background scripts UI but now that macro only allows XML files. It's possible I suppose that historically uploaded Instance Scripts may still be available or perhaps SNow themselves may have a mechnism, however as the files don't align with existing security rules it's unlikely they would provide any support in getting it working again.

There are historical articles that are not quite correct in their assumptions around how data was stored also, therefore they aren't much use in determining a potential mechanism for recovering this feature (v_customer_uploads is listed in them as where the files are stored, whereas it's a virtual table and only references the actual location).

Perhaps this is something @arnoudkooi might be able to provide in a future snutils version as it's a simple DOM element with a UL within. It would just take creation of a new tab along with a local script storage mechanism, the downside of course is that each script would only be available on a per-user basis.