However, we use nesting a lot, and we would want listed associations to include an "artifact path" attribute, and including that artifact path in a column might quickly enlarge the amount of data to be displayed past the point of what is useful.

As an alternative to listing the associated items themselves in a column, perhaps each list view row could have some kind of numerical indicator that indicates the number of associated artifacts for each artifact type.  So, for example, requirement list rows could have little icons on them.  A row could have a little "T5" icon if the requirement has 5 tasks associated with it.  Then if the user clicks on the T5 icon, a sub-grid would open up below the requirement row and the sub-grid would display the five related tasks.  (This could be infinitely nested, because the tasks would themselves would have icons for their associations.)