This is normally managed by RecentCollectionAction, but is only updated by StandardActionManager when its standard actions are used. If a caller uses its own actions, for example KMail's separate message reader window, then the list is not updated. No access is available from outside other than via those StandardActionManager actions.
Add a class-static function StandardActionManager::addRecentCollection(), which simply calls RecentCollectionAction::addRecentCollection() to update the global recent collection list. Read and update this list in RecentCollectionAction as before.