There is no way to add arbitrary metadata to the corkboard, all of the options available for doing so are located in the View ▸ Corkboard Options menu, and illustrated in the user manual with figure 8.15, in §8.2, The Corkboard.
By the way, it doesn’t really change any of the above, but have you considered using Document Bookmarks for this, rather than hard text typed into metadata fields that you have to manually look up? If you bookmark both the in and out, then automatically those scenes will backlink to this one, reducing your work effort considerably, and the result will be to provide the full text of these related scenes in the Inspector sidebar.
The only downside with Bookmarks is that you can’t annotate them, really the only “commentary” we can have about them is their order in the list. So while that does allow for a simple IN/OUT convention, it’s a bit fragile, and doesn’t announce that it is broken if new bookmarks somehow end up in between the first two or something.
So another approach is inline annotations at the top and bottom of the file, with the document link in the editor. It’s not as useful as a bookmark in terms of real-time referencing of the original text, but you can at least talk about the link around it, within the inline annotation.
I do also wonder whether that technique is necessary with Scrivener at all. To be perfectly clear, I haven’t read it and I know nothing about it, so I might be utterly mistaken, but just looking at the described result and guessing at the intent: it’s for writers that use word processors / typewriters / etc. that need to more easily get a sense of how a non-linear multi-plot book is put together. One way of doing that is by very carefully constructing a chain of links from one scene to the next, within a plot line.
But with Scrivener you can merely flag the 18 scenes as being related to a certain subplot with something like a Label, and then later search and gather all labelled scenes, viewing them in order with Scrivenings mode. You don’t need a prompt to get from scene 27 to 39 if they are right next to each other in the text editor.