Help to optimize subpartition

[Paki Paki]

I have an actively used big table with 500M records in it:

CREATE TABLE IF NOT EXISTS `bigtable` (
`userid` int(11) unsigned NOT NULL,
`status` tinyint(1) unsigned NOT NULL DEFAULT '0',
`dt` int(11) DEFAULT NULL, -- represents timestamp
KEY `dt` (`dt`),
KEY `uf` (`userid`,`fdt`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 ROW_FORMAT=DYNAMIC
PARTITION BY RANGE (dt)
SUBPARTITION BY HASH (userid)
SUBPARTITIONS 96
(PARTITION p2000 VALUES LESS THAN (946684800) ENGINE = MyISAM,
PARTITION p2011q2 VALUES LESS THAN (1309478400) ENGINE = MyISAM,
PARTITION p2011q3 VALUES LESS THAN (1317427200) ENGINE = MyISAM,
PARTITION p2011q4 VALUES LESS THAN (1325376000) ENGINE = MyISAM,
PARTITION p2012q1 VALUES LESS THAN (1333238400) ENGINE = MyISAM,
PARTITION p2012q2 VALUES LESS THAN (1341100800) ENGINE = MyISAM,
PARTITION p2012q3 VALUES LESS THAN (1349049600) ENGINE = MyISAM,
PARTITION p1 VALUES LESS THAN MAXVALUE ENGINE = MyISAM)

I need to optimize table after periodical bulk data deletions.
I can execute 'ALTER TABLE `bigtable` OPTIMIZE PARTITION `p2000`;' to optimize one partition. But it can take a long time and cause some blocking issues.

So I need to optimize table by subpartitions. Please help with ALTER command for optimize any subpartition.
Command 'ALTER TABLE `bigtable` OPTIMIZE PARTITION `p2000sp1`;' doesn't work.