I think this is done: changeset:   8797:f67948a018ec

This is what I have implemented:

  if a new item is created and a property with a default_value is not
set,
    the value in the database will be set to the default value.

  if a user then deletes/clears the value to None, it will unset the
value.
    An auditor needs to be written to block the change or force the
default value.

  if an item is edited in the web interface and the property with the
default_value is
    unset (None), the html interface will display the default value.

    This can lead to a case where the html interface displays a value
that is not
    present in the database on edit. This might have been present since
    changeset:   4481:9bbf3758c16a in 2011. But it might have been
hidden until
    issue2551132 was fixed. I'm not sure. The rest interface returns the
true db value.

I documented some caveats to using default_value and suggested using
auditors to enforce
default value. 

Also TIL that MultiLinks don't support default_value.