sys-apps/preload should come with a systemd unit. I have not personally tested the one in the user/systemd overlay, available here: http://git.overlays.gentoo.org/gitweb/?p=user/systemd.git;a=blob;f=sys-apps/systemd-units/files/services-desktop/preload.service;h=ef21efcd228f5eec1dcd09b6ed31cc64efc9bc9e;hb=3813c46a98a986050195498c6f6af6cad76982e2
In this exact case, I think people running systemd should move to its own readahead daemon (it's still maintained, while preload is dead for a long time)