Here's a way but unfortunately it isn't a TTW approach, which means creating a custom add-on, which certainly defeats the convenience of what you're trying to achieve see: http://stackoverflow.com/questions/6662467/how-do-you-override-the-default-value-of-a-field-in-a-dexterity-behavior-in-plon
OR
You could do something more "hacky" by defining fields yourself and disabling the default behaviour, at least it would still be through the web.