How can I get MySQL to use the value returned by a function as a default? I have a pre-existing table that has a column to that can hold a UUID.

I'd like to be able to define a default such that new rows coming in (from legacy systems) will still get a UUID defined, via the UUID() function -- is it possible to configure the default of a column to be a value returned by UUID (in a similar manner to how you can for the UNIX_TIMESTAMP() in phpMyAdmin)
Using a trigger

You can't use functions as defaults in MySQL, except for CURRENT_TIMESTAMP for TIMESTAMP columns