I figured out how to use a PostgreSQL CTE as part of an update statement in a Django data migration. The trick here is mainly understanding how to combine CTEs with a PostgreSQL update - here's the pattern for that:
\nfrom django.db import migrations\n\nSQL = \"\"\"\nwith created_at_by_reversion as (\n select\n location.id as id, min(date_created) as created_at\n from location\n join reversion_version on (location.id = reversion_version.object_id::integer and reversion_version.content_type_id = 18)\n join reversion_revision on reversion_revision.id = reversion_version.revision_id\n group by location.id\n),\ncreated_at_by_source_location as (\n select\n location.id as id, min(source_location.created_at) as created_at\n from source_location\n join location on source_location.matched_location_id = location.id\n group by location.id\n),\nnew_created_at_for_locations as (\n select\n location.id,\n created_at_by_reversion.created_at as created_at_by_reversion,\n created_at_by_source_location.created_at as created_at_by_source_location,\n coalesce(created_at_by_reversion.created_at, created_at_by_source_location.created_at) as new_created_at\n from location\n left join created_at_by_source_location on created_at_by_source_location.id = location.id\n left join created_at_by_reversion on created_at_by_reversion.id = location.id\n)\nupdate location\n set\n created_at = new_created_at_for_locations.new_created_at\n from\n new_created_at_for_locations\n where\n location.id = new_created_at_for_locations.id\n\"\"\"\n\n\nclass Migration(migrations.Migration):\n\n dependencies = [\n (\"core\", \"0132_location_created_at_created_by\"),\n ]\n\n operations = [\n migrations.RunSQL(\n sql=SQL,\n reverse_sql=migrations.RunSQL.noop,\n ),\n ]