Você pode dar uma olhada na estrutura de controle 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;
...
