As per this StackOverflow question:

http://stackoverflow.com/questions/3056809/how-to-manually-create-a-dbref-using-pymongo/3059200#3059200

It would be nice to be able to add additional fields to a DBRef to cache certain values from the document it references.

Having the additional values passed as kwargs to the DBRef constructor would probably be the best way to achieve this, but having access to an "extra_fields" attribute wouldn't hurt either.