This shows how to use CakePHP Expandable behavior with single table and make kind of polymorphic association. This uses a little bit modified version of expandable behavior so that it automatically adds model name to correct field for the associations.

custom_fields.sql

CREATE TABLE IF NOT EXISTS `cms_custom_fields` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `entity_id` int(11) DEFAULT NULL,
  `key` varchar(100) DEFAULT NULL,
  `val` blob,
  `model` varchar(100) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `ENTITY_ID` (`model`,`entity_id`,`key`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;

CustomField.php

<?php
class CustomField extends AppModel {
    var $name = 'CustomField';
    var $actsAs = array('Search.Searchable');
    public $filterArgs = array();
}

expandable.php

<?php

/**
 * undocumented class
 *
 * @package default
 * @access public
 */
class ExpandableBehavior extends ModelBehavior{
  var $settings = array();

	function setup(&$Model, $settings = array()) {
		$base = array('schema' => $Model->schema());
		if (isset($settings['with'])) {
			$conventions = array('foreignKey', $Model->hasMany[$settings['with']]['foreignKey']);
			return $this->settings[$Model->alias] = am($base, $conventions, $settings);
		}
		foreach ($Model->hasMany as $assoc => $option) {
			if (strpos($assoc, 'Field') !== false) {
				$conventions = array('with' => $assoc, 'foreignKey' => $Model->hasMany[$assoc]['foreignKey']);
				return $this->settings[$Model->alias] = am($base, $conventions, !empty($settings) ? $settings : array());
			}
		}
	}

	function afterFind(&$Model, $results, $primary) {
		extract($this->settings[$Model->alias]);
		if (!Set::matches('/'.$with, $results)) {
			return;
		}
		foreach ($results as $i => $item) {
			foreach ($item[$with] as $field) {
				$results[$i][$Model->alias][$field['key']] = $field['val'];
			}
		}
		return $results;
	}

	function afterSave(&$Model) {
		extract($this->settings[$Model->alias]);
		$fields = array_diff_key($Model->data[$Model->alias], $schema);
		$id = $Model->id;
		foreach ($fields as $key => $val) {
			$field = $Model->{$with}->find('first', array(
				'fields' => array($with.'.id'),
				'conditions' => array($with.'.'.$foreignKey => $id, $with.'.key' => $key),
				'recursive' => -1,
			));
			$Model->{$with}->create(false);
			if ($field) {
				$Model->{$with}->set('id', $field[$with]['id']);
			} else {
				$Model->{$with}->set(array($foreignKey => $id, 'key' => $key));
			}
			$Model->{$with}->set('val', $val);
			$Model->{$with}->set('model', $Model->name);
			$Model->{$with}->save();
		}
	}
}

Place.php

<?php
class Place extends AppModel {
    public $name = 'Place';

    public $belongsTo = array(
      'Location'
    );

    public $hasAndBelongsToMany = array(
		'Expedition'
	);
  
  // Here's the important part hasMany for Expandable to work properly
	public $hasMany = array(
		'CustomField' => array(
				'foreignKey' => 'entity_id',
				'conditions' => array('CustomField.model' => 'Place'),
				'dependent' => true
		),
	);

	public $actsAs = array(
		'Expandable',
		'Utils.Sluggable' => array(
			'label' => 'name',
	        'unique' => false,
	        'separator' => '-',
	        'length' => 100,
	        'update' => true,
	    )
	);

	function beforeSave() {
		// need to just count and update the name count
		if (isset($this->data[$this->name]['prisoner_names'])) {
			$names = explode(',', $this->data[$this->name]['prisoner_names']);
			$count = 0;
			if (!empty($names[0])) {
				$count = count($names);
			}
			$this->data[$this->name]['prisoner_name_count'] = $count;
		}
		return true;
	}

}