Using a debian-esque style like to access pgo like http://packages.gentoo.org/category/name would make it very fast and easy to access packages you know the full name of. The naming structure of portage is perfectly suited for this scheme already, so why not make use of it? Apache's mod-rewrite is my suggestion for this task. Just make it match on all category names.
Reassigning to the main infra group since Marduk is retired.
New packages2 has this.