It looks like Hibernate doesn't load up a set of defaults, but lets each bit of functionality determine the default if the property is not overridden. For example DEFAULT_ENTITY_MODE is POJO, but the only way to find that is to search everywhere for the property and trace back to what happens if the property is not assigned.
The source to SettingsFactory uses defaults on a lot of PropertiesHelpers.get... calls. The buildSettings method in that class has a lot of what you need. Also, dialects set some other defaults using getDefaultProperties().setProperty...