Fields from shared CCK fields not migrated from versions <= D6

I'm working on a D6->D7 migration, and I'm running into an issue where certain CCK fields are not imported into my D7 site, even though the mapping has been specified in my class constructor and the Migrate UI screens show the mapping as properly set up. After debugging,looking in my prepareRow() method, and combing through lots of code (including D6 CCK code), I think I've found the problem; migrate_d2d does not properly migrate from CCK fields that are shared among multiple content types. The TL;DR version is that it chooses for some reason not to join on content_field_* tables for commonly shared fields.

For those who don't understand what I mean, in D6 and older, if you create a custom CCK field and only add it to one content type, the columns for that field are put in the same table as the content type (content_type_*). However, once that field is used in multiple content types, a separate table is created for it (content_field_*), and the columns for that table are removed from the content_type_* table. This means that if you want the data for that field, you have to join on the content_type_* table to the content_field_* in nid and vid (this is corrected in D7 in that all fields get their own custom table - field_data_* - regardless of whether or not they're only used in one content type).

The code that is causing this is in the various query() methods (e.g. DrupalNode6Migration::query(), DrupalEntity6Migration::query(), DrupalVersion6::getSourceValues()):

      foreach ($field_info as $field_name => $info) {
        if (isset($info['columns']) && !$info['multiple'] && $info['db_storage']) {

The code only goes into that loop if db_storage == 1, and there is no else statement. The problem is, db_storage = 1 (in the content_node_field table) only for fields whose data is stored in the content type base table. Therefore, the join is never added to the query, and the data for that field is never obtained.

My fix for this is to add a query() override in my migration class to get this data (this is for a CCK field called field_source):

  protected function query() {
    // Get the default query.
    $query = parent::query();

    // Add a join to content_field_video.
    $query->rightJoin('content_field_source', 'cfs', 'cfs.nid=nr.nid AND cfs.vid=nr.vid');
    // Add the condition for the eyeblast provider.
    $query->fields('cfs', array('field_source_nid'));

    return $query;
  }

along with a prepareRow() method to put the data in the expected variable name:

  function prepareRow($row) {
    $row->field_source = $row->field_source_nid;
  }

Although I suppose another way would be to define field_source_nid as a custom source field and then the prepareRow() stuff wouldn't be necessary, but it seems like six of one and half dozen of another to me.

So for migrate_d2d gurus, am I reading this correctly? Is this a bug, or is there an existing way to get data from commonly shared fields in D6 and older that I'm missing?

Thanks.