[Radiant] change default values

[Francesco Lunelli]

I need to change the default values inserted by radiant in published_at
column.
In the old version of Radiant I created sites with all controllers,
models etc in the site so I modified them to avoid that the default
values was inserted in the db.
But now I migrated to the new version of Radiant 0.6.1 and all the code
of Radiant is in the gem. How can I ovewrite the default functionality
and insert my values in published_at column?
Thanks in advance

-- 
Francesco Lunelli
Ymir s.r.l.
Viale Verona 190/11
38100 Trento