Obviously, modifying these files is not ideal and without some kind of conditionals you won't be able to reuse playa field types for anything else, but if you must here is what I did.
The 3 files that I needed to modify:
ft.playa.php in the
display_field method you need to change the query:
$query = $this->EE->db->query(
'SELECT * FROM exp_channel_titles
WHERE entry_id '.$this->helper->param2sql($vars['selected_entry_ids']));
to join on the
exp_channel_data table and select your custom field. This is for your saved selected item(s).
helper.php in the
entries_query method you need to change the query being built in a few places to join on the
exp_channel_data table and select your field. This is for the available unselected options. Also, in the same method under
if (isset($params['keywords']) && $params['keywords'])
Finally, you need to print your custom field in the