> One could write a dual-portage thingy that replicates what you have then
> does emerge --sync, and also has an --undo fetaure for just in case.

Or you could simply use squashfs + aufs/unionfs to host your portage
tree, see http://forums.gentoo.org/viewtopic-t-465367.html.
This way you can still access files from the previous portage tree
after syncing without any problems (i.e. without unpacking any backup).
Moreover, since the portage tree will get recompressed on shutdown
anyway, it does not cost additional disk operations to keep
backups for as many revisisions as you want to.
--
gentoo-user@(protected)