At first glance, it seems that you should be able to do this relatively simply by modifying the metawidget XML configuration, and perhaps providing your own generation class, but I will need to defer you to Richard Kennard, the author of Metawidget, since he will know the answer (and hopefully be able to tell you how to proceed without too much trouble on his part.)
I am not too knowledgeable about this part of Forge. I've also copied a few people who might be able to give advice with their experience writing custom scaffolds.