Will take chance for question here.

We ran into similar scenario as well when we designed virtualized Gentoo
infrastructure and we found out that emerge/portage has not complete (read
perfect) support for binaries (i.e. different USE flags for different
machines).

Can you explain more about your project? It seems it really fits our needs.
Some of our thoughts were to have this builder machine available as XEN
domU, it would build the packages in chroot and then serve them to machines
url  as:

PORTAGE_BINHOST="http://packages.builder.domain/<machine-type>/"

Some other crazy ideas involved some extension to emerge/portage like:
Slave - hey, this is my /var/lib/portage/world, my USE flags
(/etc/make.conf) and I want such and such package
Master - calculating a bit and providing Slave with packages

(but this is overkill)

At the moment I have working prototype which is fine because machines are
updated and it is doable, at the same moment it needs a lot of love because
it is just a dirty hack (like scripts to easily start chroot, config apache,
etc...).

I do not know catalyst much, all I know it is possible to do stage4 with
this (i.e. build up2date XEN domu machine). I appreciate any hints on how to
incorporate this into something slightly bigger.

I am looking for anyone interested in this, lets join effort guys.

Aloha,

Robert