//mods:mods/mods:subject/mods:title

As a stop-gap measure to avoid having to reindex the entire dataset (and until Ingest gets partial indexing logic), after adding the xpath setting above:

-- insert into metabib.subject_field_entry (field, source, value)
SELECT
    -- (select id from config.metabib_field where field_class = 'subject' and name = 'title'),
    record,
    array_to_string( array_accum( VALUE ), ' ' )
  FROM 
    metabib.full_rec
  WHERE
    tag='630'
    AND subfield IN ('a','d','f','h','k','l','o','r')
  GROUP BY 1 --, 2
;

Just uncomment the hidden bits to perform the insertion after testing.