Plugin is a widget, but I want to call it in the head, can I?

The correct “WordPress” way of doing it would be to use the_widget template tag to display widgets anywhere you want.

so in you case you would something like this:

$instance['account'] = 'account name'; //Ustream channel name
$instance['online'] = 'http://www.online_image.URL';
$instance['offline'] = 'http://www.offline_image.URL';

the_widget('wp_ustream_status', $instance);