I just updated a whole bunch of ebuilds for gentoo-prefix you can find the overlay here.

Just clone it and put it somewhere in your $EPREFIX/usr/local/portage then add that folder to your make.conf

In my case the make.conf looks as follows:

USE="unicode nls ruby perl xterm-color X"
USE="$USE xcb svg"
USE="$USE png jpeg mmx bzip2"
CFLAGS="-O2 -pipe -march=nocona"

Then simply doing a:

emerge hsetroot
USE=osx emerge awesome rxvt-unicode

Instead of USE=osx you could also do a:

mkdir -p $EPREFIX/etc/portage/package.use
echo 'x11-terms/rxvt-unicode osx' > $EPREFIX/etc/portage/package.use/rxvt

Anyway, that should be enough to install awesome 3.2.1 and rxvt-unicode with copy&paste support for osx.