manual execution of specific remote query with postgresql_fdw
For the time being, it seems that the best available option is to create a view on the remote server that encapsulates the query needing to be "pushed down".  postgres_fdw is happy to define and use foreign tables backed by views on the remote, and regular old query optimization within the view does the Right Thing.  For instance, given

CREATE VIEW id_ranges AS
SELECT 'url_strings'::text AS tbl,
    min(url_strings.id)::bigint AS lo,
    max(url_strings.id)::bigint AS hi
   FROM url_strings
UNION
 SELECT 'captured_pages'::text AS tbl,
    min(captured_pages.url)::bigint AS lo,
    max(captured_pages.url)::bigint AS hi
   FROM captured_pages
UNION
 -- ... several more like that ...


on the remote, and a FOREIGN TABLE of the same name on the local server,

SELECT lo, hi FROM id_ranges WHERE tbl = 'url_strings';


the existing pushdown optimization will send the WHERE constraint to the remote, and the remote will scan only one table (making use of indexes if possible) and send back a single-row result.