My try at creating the client extension (to be tested)

client/sql/0.0.1/10_admin0.sql

@@ -0,0 +1,29 @@
+--
+-- Public geocoder API function
+--
+-- These are the only ones with permissions to publicuser role
+-- and should also be the only ones with SECURITY DEFINER
+
+CREATE OR REPLACE FUNCTION geocode_admin0_polygons(country_name text)
+RETURNS Geometry AS $$
+DECLARE
+  db_connection_str text;
+  ret Geometry;
+BEGIN
+  SELECT _config_get('db_connection_str') INTO db_connection_str;
+  SELECT _geocode_admin0_polygons(session_user, txid_current(), db_connection_str, country_name) INTO ret;
+  RETURN ret;
+END;
+$$ LANGUAGE 'plpgsql' SECURITY DEFINER;
+
+
+-- TODO: review all permissions stuff [I'd explicitly grant permissions to the public functions]
+
+--------------------------------------------------------------------------------
+
+CREATE OR REPLACE FUNCTION _geocode_admin0_polygons(user_id name, tx_id bigint, db_connection_str text, country_name text)
+RETURNS Geometry AS $$
+  -- TODO check if we can move the config to its own function
+  CONNECT db_connection_str;
+  SELECT geocode_admin0(user_id, tx_id, country_name);
+$$ LANGUAGE plproxy;