Nope, the parser seems to be pretty brittle around this. Someone who knows more about the internals can probably explain why this works:

  class User
  {
      public $mapping = array(
          'username' => array(
              'type' => 'string',
              'length' => 32,
              'unique' => true,
          )
      );  

      public function __construct()
      {
      	$this->mapping['nullable'] = function() { echo "test"; };
      	$this->mapping['nullable']();
      }
  }

  $user = new User();

And this doesn't:

  class User
  {
      public static $mapping = array(
          'username' => array(
              'type' => 'string',
              'length' => 32,
              'unique' => true,
          )
      );  

      public static function set()
  	{
  		self::$mapping['nullable'] = function() { echo "test"; };
  		self::$mapping['nullable']();
  	}
  }

  User::set();