Drush site-install working with profile_name_install_tasks and forms
Drush si command has key=valueargument.
The key is in the form [form name].[parameter name] on D7 or just [parameter name] on D6.

drush site-install standard install_configure_form.site_default_country=FR


See more: http://www.drushcommands.com/drush-6x/core/site-install