I don't know if this  will have any relevance any more as the original post is quite old.  However here goes.  I also noticed that  querying the API for custom fields does not  return  them  unless they are specified in the  return fields.  But the field that is  listed in the API explorer in not the actual field name,  I could not get the  fields  to return using their "title names"  but they do when using the custom_xx  field name.