Migration::applyMappings() unable to handle multifield subfields

While working on a migration with an image field inside of a Multifield I've been struggling with an issue with migrating to the image field. The full description of the problem is in that issue, but here it is again.

In using a multifield, if you want to set a field subclass (e.g. file_class, preserve_files, source_type, etc), you have to do it in three levels:

    $this->addFieldMapping('field_images:field_image', 'field_image')
      ->sourceMigration('CNSFile');
    $this->addFieldMapping('field_images:field_image:file_class')
      ->defaultValue('MigrateFileFid');
    $this->addFieldMapping('field_images:field_image:preserve_files')
      ->defaultValue(TRUE);

However, the code in Migration::applyMappings() doesn't handle this because it uses hardcoded values:

        // Are we dealing with the primary value of the destination field, or a
        // subfield?
        $destination = explode(':', $destination);
        $destination_field = $destination[0];
        if (isset($destination[1])) {
          $subfield = $destination[1];
          // We're processing the subfield before the primary value, initialize it
          if (!property_exists($this->destinationValues, $destination_field)) {
            $this->destinationValues->$destination_field = array();
          }
          // We have a value, and need to convert to an array so we can add
          // arguments.
          elseif (!is_array($this->destinationValues->$destination_field)) {
            $this->destinationValues->$destination_field = array($this->destinationValues->$destination_field);
          }
          // Add the subfield value to the arguments array.
          $this->destinationValues->{$destination_field}['arguments'][$subfield] = $destination_values;
        }
        // Just the primary value, the first time through for this field, simply
        // set it.
        elseif (!property_exists($this->destinationValues, $destination_field)) {
          $this->destinationValues->$destination_field = $destination_values;
        }
        // We've seen a subfield, so add as an array value.
        else {
          $this->destinationValues->{$destination_field} = array_merge(
            (array)$destination_values, $this->destinationValues->{$destination_field});
        }

Since it only looks for $destination[0] and $destination[1] (and doesn't look to see if there is $destination[2]), when it gets to this line:

 // Add the subfield value to the arguments array.
  $this->destinationValues->{$destination_field}['arguments'][$subfield] = $destination_values;

it overwrites the desired data value in $this->destinationValues->{$destination_field}['arguments'][$subfield] ($this->destinationValues->{'field_images'}['arguments']['field_image] in my case) with the subclass value, in my case overwriting a file id first with the MigrateFileFid class name, and then preserve_files value (true). So, when it gets to the multifield migration handler, the field has a value of true instead of the fid, that part of the migration fails, and I get an SQL error about inserting a blank value into the file_usage.fid field.

So as I see it, this method could be improved by being able to handle more than two levels of values in the field mappings for cases like this. Assuming that is ok, I'd be more than happy to write a patch, but if not, I'm open to suggestions.

Thanks.