I haven't actually tried this, and I'm not sure if Database Script events work
on the web, but it would work if you have a web front-end for users, and a
Notes version for content maintenance (we have a number of apps like this.) In
the QueryDocumentDelete event, create a documentcollection of all docs that
contain the ID of the document being deleted, then loop through those and
remove the ID from the field.
You could also accomplish the whole related links thing a different way,
although it's not really the point of your article. Create a view categorized
by the multivalue ID field, displaying multiple values as separate entries.
Then, in your form, have an embedded view, single category which is the ID of
the current document.