Answer
Add the custom field to the ShipmentSourceUtility entity instead — that's the underlying entity that actually surfaces on the Available Lines for Shipments screen, even though SourceLinesAvailableForShipment itself isn't directly configurable for custom fields. No additional view configuration is needed once the field is on ShipmentSourceUtility. One limitation to note: a custom field added this way only supports read-only, not editable, behavior on that screen.