not quite "just" an overlay, but yes you can start out with a gentoo system and then add the sabayon overlay, and emerge from there. A word of caution, a lot of the OOTB experience comes from kernel tweaks and other tweaks to the gentoo system that you may not get without installing our custom kernel. One of the "scandalous" configurations that SL uses is ACCEPT_Keywords="~x86". So you will get a lot of unstable packages in there. There has been a lot of discussion in the development forum about how to take an SL system and hack it down from the "~x86" branch to a stable system. Check out the development forum to see what other SL/gentoo users have come up with to get SL but keep more of a stable Gentoo base system.
Short answer, yes we just use portage with our custom tweaks in our overlay.