Add custom data field column to relationship tab
Some of the pages that you would expect to support hook_civicrm_searchColumns don't - they're mostly older screens that don't use the appropriate shared code.

Ideally, we would rewrite these pages to use CRM_Core_Selector_Query so the hook fires.  Since that's not often feasible within a budget, I've resorted to using template overrides, with API calls within the override to pull in additional data.

If you want to avoid overrides, you could probably use JavaScript includes.

None of these are perfect, but short of rewriting the screens (a noble goal!) this is what we have to work with!