Allow comments without approval for custom content type

Yes, it is possible using wp_insert_comment hook. To approve comments automatically for custom post types you can use something similar to this:

//hook to do stuff with comments
add_action( 'wp_insert_comment', 'approve_comment', 99, 2 );

function approve_comment($comment_id, $comment_object) {
   //get id of the post that was commented
   $post_id = $comment_object->comment_post_ID;

   //approve comment for "post" type
    if( 'post' == get_post_type( $post_id )) {
       $retrieved_comment = get_comment( $comment_id, ARRAY_A );
       //approve comment    
       $commentarr = $retrieved_comment;
       $commentarr['comment_ID'] = $comment_id;
       $commentarr['comment_approved'] = 1;
       wp_update_comment( $commentarr );
   }
}