I found the nifty topic_list[offset-id] parameter to scroll a particular topic into view. I think it takes the UUID of the previous workflow.
If you get it wrong as in the URL above, you see an empty browser window, and error.log contains
[Tue Dec 10 08:57:17 2013] [error] [client 188.8.131.52] PHP Catchable fatal error: Argument 1 passed to Flow\\Model\\TopicListEntry::toStorageRow() must be an instance of Flow\\Model\\TopicListEntry, null given in /srv/mediawiki/extensions/Flow/includes/Model/TopicListEntry.php on line 32
If you just give a garbled parameter like http://ee-flow.wmflabs.org/wiki/Talk:First_release?topic_list[offset-id]=bad , you at least get an exception error reported to the browser:
[1a4011a3] /wiki/Talk:First_release?topic_list[offset-id]=bad Exception from line 48 of /srv/mediawiki/extensions/Flow/includes/Model/UUID.php: Unknown input to UUID class
Maybe Flow should support a general query string parameter to make visible any topic or post, to fix bug 58251