I can be a bit dense sometimes. Splitting out the portage directories onto a separate disk allows users to skip downloading them if they don't want them. Splitting off the Sugar directories would allow the developer to update to a new Sugar "release" with just a few tens of megabytes more. That would let developers keep up-to-date without needing to work with sugar-jhbuild build.
I'd been thinking of doing the install as an ebuild for the Gentoo machine, but that won't work when we move to an FC7 image.
[Update] Hitting the "slow down" button on the Developer Image creation stuff for right now. The Tuquito group is apparently producing a LiveCD that uses AUFS to let you run the developer's image from the CD while writing to a USB or filesystem. More information as that develops.