package: Allow setting debconf selections before package is installed?

When ejabberd and jwchat are setup by freedombox-setup, some debconf selections are set before the packages are installed: https://github.com/petterreinholdtsen/freedombox-setup/blob/master/setup.d/85_jwchat#L8

I'm thinking that it would be useful if package.py allowed debconf selections to be specified somehow in the @package.required decorator.