There **is** a solution with just PL/pgSQL. Simple and elegant, too. Pretty advanced stuff, though.  
Requires Postgres **9.0** or later (workaround for older versions possible).

    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
    
       EXECUTE format('INSERT INTO %s SELECT $1.*', _tbl_out)
       USING _tbl_in;
    END LOOP;
    
    END
    $func$  LANGUAGE plpgsql;

Call (important!):

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

`t` and `t1` being the tables with identical schema.

**Note** that the polymorphic parameter (`anyelement`) is only needed if you need it's value or data type for the computation in the function body. Else you can simplify like demonstrated in this later answer:

- https://dba.stackexchange.com/questions/163962/insert-values-from-a-record-variable-into-a-subclass-table/164224#164224

###Major ingredients

- [Implicit cursor of a `FOR` loop][1] instead of explicit cursor. That's generally preferable.

- [Polymorphic types][2]

- [Object identifier types][3]

- [Dynamic SQL in plpgsql][4]

- [`VALUES` can take a row type directly.][5]

An obstacle to overcome is that variables inside the function cannot be defined as polymorphic type `anyelement` (yet). [This related answer on SO][6] explains the solution. Provides a **workaround for older versions**, too.

I am handing in a `NULL` value of type `t`, which serves three purposes:

- Provide table name.
- Provide table type.
- Serve as loop variable.

The *value* of the first parameter is *discarded*. Use `NULL`.

Consider [this related answer on SO with more details][7]. The most interesting part being the last chapter **Various complete table types**.

[**SQL Fiddle demo.**][8]

If your computations are not too sophisticated, you may be able to replace the loop with a single dynamic SQL statement, which is typically faster.


  [1]: http://www.postgresql.org/docs/current/interactive/plpgsql-control-structures.html#PLPGSQL-RECORDS-ITERATING
  [2]: http://www.postgresql.org/docs/current/interactive/extend-type-system.html#EXTEND-TYPES-POLYMORPHIC
  [3]: http://www.postgresql.org/docs/current/interactive/datatype-oid.html
  [4]: http://www.postgresql.org/docs/current/interactive/plpgsql-statements.html#PLPGSQL-STATEMENTS-EXECUTING-DYN
  [5]: https://stackoverflow.com/questions/23247105/postgresql-9-3-trigger-function-to-insert-into-table-with-parameterized-name/23249595#23249595
  [6]: https://stackoverflow.com/questions/7634704/declare-variable-of-composite-type-in-postgresql-using-type/7635605#7635605
  [7]: https://stackoverflow.com/questions/11740256/refactor-a-pl-pgsql-function-to-return-the-output-of-various-select-queries/11751557#11751557
  [8]: http://www.sqlfiddle.com/#!15/998bb/2