[gentoo-user] Update gentoo from live usb?

[Andreas Fink]

Hello,

I have a couple of systems that I do not update regularily (some not
even for years). But then sometimes I feel, hey I should do an update.
I have one master build server which builds packages and keeps them as
binary packages, annd all my systems pull the gentoo portage tree from
this master build server, additionally also the configs in /etc/portage
is the same amongst all boxes, i.e. use flags et al are all the same.

Now comes the misery when I want to update an old box, because of
unsupported EAPI and what not. One way that I used in the past was to
extract a stage-3 tarball over the existing root system, and then do
the upgrade, which works to some extent, but it does not seem right.
Coming now to my question: Is it possible to start a live gentoo system
with a recent portage version and then tell portage that it should
install the packages in /mnt/gentoo (which is the real system I care
about). I have heard about the prefix project, but I'm not sure if this
is exactly what I want.
Maybe a second approach would be to get the minimal set of binary
packages from the master build server and extract them manually, such
that I end up with a recent enough portage which supports all EAPIs
that are in the tree. But I have no clue how to get the minimal set of
packages that I would need to extract.

Does anybody have other approaches (besides starting from scratch)?