Omit single post from the loop using pre_get_posts ?

I’d like to omit posts with the custom field meta “locked=1” from the loop. I toyed around with using the pre_get_posts filter based on a similar usage for excluding categories such as:

function myFilter($query) {
	if ($query->is_feed) {
		$query->set('cat','-5');
	}

return $query;
}
add_filter('pre_get_posts','myFilter');

My goal is to be able to omit from is_feed any posts containing the custom field “locked” with any non null value. Without too much knowledge of how filters work in WordPress, I tried the code:

function myFilter($query) {
	if ($query->is_feed)
		if (get_post_meta($query->query_vars['page_id'], 'locked', TRUE))
			$query->set('page_id','-'.$query->query_vars['page_id']);
	return $query;
}

To no avail. I know I’m probably way off track but just can’t seem to get my head around it tonight. Is this functionality possible or am I dreaming? Is there a better way to achieve this? I’m using WP 2.3.3.

Thanks in advance!