Schedule future post to custom post status instead of publish?

Here is one idea:

You could try to use the future_to_publish action to change the post status:

add_action('future_to_publish', 'set_status_online_wpse_95701'); 
function set_status_online_wpse_95701( $post ) { 
    if ( $post && $post->post_type==="stream"){
       $post->post_status="online"; // change the post_status
       wp_update_post( $post );
    }    
}