Issues with optimizing 'ORDER BY' inside a 'UNION' query

OrderByOptimization.sql

EXPLAIN EXTENDED 
SELECT l.id, l.account_id, l.vnum, l.count, l.preis, l.zeitpunkt AS zeit,
 'hp' AS source FROM is_log AS l WHERE l.account_id = 730
 ORDER BY l.zeitpunkt DESC LIMIT 10;


+----+-------------+-------+-------+---------------+---------------+---------+------+------+----------+-------------+
| id | select_type | table | type  | possible_keys | key           | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+-------+---------------+---------------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | l     | index | NULL          | idx_zeitpunkt | 8       | NULL |   10 |    17170 | Using where |
+----+-------------+-------+-------+---------------+---------------+---------+------+------+----------+-------------+
1 row in set

TableStructure.sql

DESCRIBE is_log; -- same for 'ig_is_log'


+------------+------------------+------+-----+---------+----------------+
| Field      | Type             | Null | Key | Default | Extra          |
+------------+------------------+------+-----+---------+----------------+
| id         | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| account_id | int(10) unsigned | NO   |     | NULL    |                |
| vnum       | int(10) unsigned | NO   |     | NULL    |                |
| count      | int(10) unsigned | NO   |     | 1       |                |
| preis      | int(10) unsigned | NO   |     | NULL    |                |
| zeitpunkt  | datetime         | NO   | MUL | NULL    |                |
+------------+------------------+------+-----+---------+----------------+
6 rows in set

UnionOrderByOptimization.sql

EXPLAIN EXTENDED (SELECT l.id, l.account_id, l.vnum, l.count, l.preis, l.zeitpunkt AS zeit,
 'hp' AS source FROM is_log AS l WHERE l.account_id = 730
 ORDER BY l.zeitpunkt DESC LIMIT 10)

UNION

(SELECT l.id, l.account_id, l.vnum, l.count, l.preis, l.zeitpunkt,
 'ig' AS source FROM ig_is_log AS l WHERE l.account_id = 730
 ORDER BY l.zeitpunkt DESC LIMIT 10)

ORDER BY zeit DESC LIMIT 10;


+------+--------------+------------+------+---------------+------+---------+------+------+----------+-----------------------------+
| id   | select_type  | table      | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                       |
+------+--------------+------------+------+---------------+------+---------+------+------+----------+-----------------------------+
|    1 | PRIMARY      | l          | ALL  | NULL          | NULL | NULL    | NULL | 1717 |      100 | Using where; Using filesort |
|    2 | UNION        | l          | ALL  | NULL          | NULL | NULL    | NULL |   23 |      100 | Using where; Using filesort |
| NULL | UNION RESULT | <union1,2> | ALL  | NULL          | NULL | NULL    | NULL | NULL | NULL     | Using filesort              |
+------+--------------+------------+------+---------------+------+---------+------+------+----------+-----------------------------+
3 rows in set