What I ended up doing was just updating our production DB, replacing all the
NULLs with the desired default value.

> We use the database's actual values when we search. The SQL (or equivalent on nosql
> backends) would have to change to include 'newprop IS NULL' in addition to any other
> criteria for properties with a default_value if the user is searching for the
> default_value.

This is indeed a valid concern. As you say, this is not straightforward and 
probably very difficult to accomplish without breaking changes.