multiple database relationship on field other than primary key in CakePHP

I have a project that necessarily spans several databases.

One database has tables:

CREATE TABLE device {
  device_uid integer unsigned not null primary key auto_increment,
  os varchar(50),
  name varchar(50)
}

CREATE TABLE platform {
  platform_id integer unsigned not null primary key auto_increment,
  name varchar(50)
}

The other database has the table:

CREATE TABLE oses_platforms {
  platform_id integer unsigned not null primary key auto_increment,
  os varchar(50),
  platform_id integer unsigned
}

I've created Models for the tables and the relationship:

<?php
class Platform extends AppModel {

    var $name = 'Platform';
    var $useTable = 'platform';
    var $primaryKey = 'platform_id';
    var $useDbConfig = 'otherdb';

    var $hasOne = array(
        'OsesPlatform' => array(
            'className' => 'OsesPlatform',
            'foreignKey' => 'platform_id'
        )
    );
}

class Device extends AppModel {

    var $name = 'Device';
    var $primaryKey = 'device_uid';
    var $useDbConfig = 'otherdb';        

}

class OsesPlatform extends AppModel {

    var $useDbConfig = 'default';
    var $name = 'OsesPlatform';
    var $primaryKey = 'os';


        var $belongsTo = array(
                'Platform' => array(
                        'className' => 'Platform',
                        'foreignKey' => 'platform_id',                            
                ),
        );

        var $hasMany = array(
                'Device' => array(
                        'className' => 'Device',
                        'foreignKey' => 'os',
                        'dependent' => false,                            
                )
        );
}
?>

If all three tables resided in 'default' or 'otherdb', I could do it in the 'conditions' argument to a hasOne or belongsTo relationship from Device to OsesPlatform, and in fact the hasOne relationship between Platform and OsesPlatform works fine. However, it's the relationship between Device and Platform that I need to model.