If not, would this work (postgres datastore):

sql
SELECT
  CASE
    WHEN userset_relation = '...' THEN
      CONCAT("namespace", ':', object_id, '#', relation, '@', userset_namespace, ':', userset_object_id)
    ELSE
      CONCAT("namespace", ':', object_id, '#', relation, '@', userset_namespace, ':', userset_object_id, '#', userset_relation)
  END
FROM relation_tuple
WHERE deleted_xid = '9223372036854775807'::xid8;

Would this WHERE condition do what I want? I don't necessarily need perfectly up-to-date relations (it'd be outdated in seconds anyway), just vaguely up-to-date. Performance isn't really a concern, I'd run it on a copy of the live database

And yes, you are correct that there is no command to export in the format that import expects.