I’m not familiar with ports, does it provide an easy way to install packages of a particular version? Is it OpenBSD only, or just a system of installing things?

OpenBSD's ports are just a collection of perl scripts and makefiles managed by a VCS (usually CVS though there are mirrors). Due to how recursive CVS works, you can easily update any part of the tree to a different commit/tag.

$ cd ${PORTSROOT}/games/stone-soup
$ cvs up -CPd -D 'some date_spec' #or -r some_tag
$ make install
$ヽ༼ຈل͜ຈ༽ﾉ enjoy your old or backported videos games

very scriptable, should be easy to figure out how to automate this should you need to.