I tried implementing it myself when the guild first went open source, and the problem with using the first post id is that the first page of results will always meet the criteria of id > afterid. I couldn't think of any ways to do it using post ids, unless you store the previous beforeid in case you need it again. Or, like previously suggested, use JavaScript back. If Mahz can figure out a way to make the database query work, then afterid will work. To Mahz, I'd suggest using a subquery sort asc, then limit outside it and reorder. Works in theory, but it wouldn't play nice while I was trying it. 

Sorry for the formatting, on mobile and just woke up.