Динамические внешние ключи - как реализовать?

You're using a design called polymorphic associations and it's frequently done wrong. The way to make it work is to create another table, say members:abstract:

CREATE TABLE IF NOT EXISTS `members:abstract` (
 `id` INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
 `type` enum('class','elected','appointed','status') NOT NULL,
  UNIQUE KEY (`id`, `type`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 COLLATE=utf8_bin;

This table serves as the parent table for all of your members attributes tables. Each of these tables changes its primary key to not generate id values automatically, but instead reference the primary key of members:abstract. I'll show just members:appointed but the others would be similar.

CREATE TABLE IF NOT EXISTS `members:appointed` (
 `id` INT UNSIGNED NOT NULL PRIMARY KEY, -- not auto_increment
 `name_prefix` varchar(8) collate utf8_bin NOT NULL COMMENT 'Prefix Added to Members Name',
 `hours_extra` decimal(4,2) NOT NULL COMMENT 'Hours Given as Bonus for Holding this Position.',
 `position` varchar(40) collate utf8_bin NOT NULL COMMENT 'Name of the Posisiton',
 FOREIGN KEY (`id`) REFERENCES `members:abstract` (`id`) ON DELETE CASCADE
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 COLLATE=utf8_bin COMMENT='Undefined within the SOP or By-Laws.';

You can make this table gain auto-generated values automatically with a trigger:

DELIMITER //
DROP TRIGGER IF EXISTS ins_appointed//
CREATE TRIGGER ins_appointed BEFORE INSERT ON `members:appointed`
FOR EACH ROW BEGIN
  INSERT INTO `members:abstract` (`type`) VALUES ('appointed');
  SET NEW.id = LAST_INSERT_ID();
END; //
DELIMITER ;

Do the same for each of the other attribute tables.

Note that the id values are now unique across all your attribute tables.

Next you make members:abstract the target for a foreign key in members_history.

CREATE TABLE IF NOT EXISTS `members_history` (
 `id` INT unsigned NOT NULL auto_increment COMMENT 'Unique Id',
 `mid` INT unsigned NOT NULL COMMENT 'Members Unique Id.',
 `value` INT UNSIGNED NOT NULL,
 `table` enum('class','elected','appointed','status') NOT NULL,
 `start` date NOT NULL COMMENT 'Value''s Effect Date',
 `end` date default NULL COMMENT 'Value''s Expiration Date',
 PRIMARY KEY (`id`),
 FOREIGN KEY (`mid`) REFERENCES `members` (`id`) ON DELETE CASCADE,
 FOREIGN KEY (`value`, `table`) REFERENCES `members:abstract` (`id`, `type`) ON DELETE CASCADE
) ENGINE=InnoDB  DEFAULT CHARSET=latin1 COMMENT='Member History';

Notice that this table defines a foreign key so that you can't reference an id of the wrong type of attribute in members:abstract.

Now you can rely on referential integrity and consistency which is impossible when you try to implement polymorphic associations without the common parent of all the referenced attribute tables.

Here's the query that returns the result you described (tested on MySQL 5.1.40):

SELECT m.username,
  m.password, m.salt, m.name_first, m.name_last,
  MAX(a.name_prefix) AS name_prefix,
  COALESCE(MAX(a.hours_extra), MAX(e.hours_extra)) AS hours_extra,
  MAX(m.date_join) AS date_join,
  MAX(m.date_leave) AS date_leave,
  MAX(a.position) AS appointed,
  MAX(c.class) AS class,
  MAX(e.position) AS elected,
  MAX(s.status) AS status
FROM `members` m 
JOIN `members_history` h ON (h.mid = m.id)
LEFT OUTER JOIN `members:appointed` a ON (h.table = 'appointed' AND h.value = a.id)
LEFT OUTER JOIN `members:class` c ON (h.table = 'class' AND h.value = c.id)
LEFT OUTER JOIN `members:elected` e ON (h.table = 'elected' AND h.value = e.id)
LEFT OUTER JOIN `members:status` s ON (h.table = 'status' AND h.value = s.id)
GROUP BY m.id;