Insert values from a record variable into a subclass table
I've realized my problem has a very simple solution. As long as we are sure about order of the schema and the extra columns are after all the original columns, my problem is simply solved by adding the values to the SELECT of the INSERT INTO

CREATE OR REPLACE FUNCTION gesio(_tbl_in anyelement, _tbl_out regclass)
  RETURNS void AS
$func$
BEGIN

FOR _tbl_in IN EXECUTE
   format('SELECT * FROM %s', pg_typeof(_tbl_in))
LOOP
   -- do something with record
   computed_column1 := 'some value';
   EXECUTE format('INSERT INTO %s SELECT $1.*, computed_column1', _tbl_out)
   USING _tbl_in;
END LOOP;

END
$func$  LANGUAGE plpgsql;


Call (important!):

SELECT gesio(NULL::t, 't1');


t and t1 are tables with identical schemas, but t1 has added a varchar column. The name of the column doesn't matter here because we're only using the order of the columns as they appear in the schemas of the two tables.