I would probably go about this way:
The plugin has a function called:
wp_ozh_yourls_send_tweet($tweet);
as you can see it accepts the tweet message and posts it to twitter,
also it has another function called:
wp_ozh_yourls_geturl( $post_id );
which accepts a post id and returns a shorturl as string.
so after knowing that I would use these two in conjunction with wordpress’s
wp_schedule_single_event
and create my own function to use the plugins functions.
Now after that is said knowing how OZH codes his plugins i bet you can find hooks somewhere in there that will make you life a lot easier.
Hope this helps.