I am messing around with Gentoo and I have never done so before but installing takes forever. So when I get a stable base establish I want to back up my Gentoo install to Ubuntu so if I mess it up I can just restore. What is the best way to do that?
My quick answer will be to just use
But, reading again, it seems that you want something like preparing your own distribution. For this you can look at Gentoo's Catalyst:
Or, better for your type of dynamic needs from the installation, there is this article: Build Your Own Live-CD or Live-DVD.