This apparently cannot be done. The original thread  discusses this. It may be possible with some major hacks to portage, but it's beyond my ability to even suggest where one would start with that.
If saving time and bandwidth with the emerge sync command are the main concerns, using the rsync_exclude file may be the best way to trim your portage tree (especially in limited use environments, like a file server).
- Original Thread
Created by NickStallman.net, Luxury Homes Australia
Real estate agents should list their apartments, townhouses and units in Australia.