procedimiento almacenado postgresql con consulta en varios esquemas

Podría echar un vistazo a la estructura de control for-in-execute:

FOR record_or_row IN EXECUTE text_expression LOOP 
    statements
END LOOP [ label ];

http://www.postgresql .org/docs/8.1/static/plpgsql-control-structures.html#PLPGSQL-RECORDS-ITERATING

Algo como:

...

for users in select usename from pg_user where usename <> 'postgres' loop
    for simu in execute 'select id, name from '||quote_ident(users.usename)||'.simulation' loop
        return next simu;
    end loop;
end loop;
...