Just make a local overlay:
http://forums.funtoo.org/viewtopic.php?id=22
Instead of running 'digest' use 'manifest' as the prior is deprecated. Copy over the POL ebuild with the same directory structure as portage (/usr/local/portage/app-emulation/playonlinux). Change the version number in the ebuild filename to the version you want to pull before you run 'manifest'. That is all that is required to pull the version you want. I just update my local ebuild every time a new version of POL is released. You have to 'manifest' each time you change it then emerge.