i never made this,since my net is fast, but i have in mind that the gentoo architeture linuxes may have dependencies when used without net..., and also Sabayon have, many, many packages.
but i suggest to you to try a simply procedure:
1) for identify, for select the packages that you want, the more easy manner is to open sulfur, it have the names of all packages.
2) create a home dir, example, /home/user (use your user name in place of "user")
2) download all the packages that you want (choosed with the aid of Sulfur) from:http://www.sabayon.org/packages
3) and save in this dir, therefore made a copy to a CD ou a DVD. in this manner you will have the packages in the CD.
if you choosed many, an CD is not sufficient to store then
4) Open Sulfur again and choose from the menu "Sulfur" "Install from Binary", and when the window open, choose to install the package from your CD or DVD.