At least chromiuos and coreos are using gentoo ebuilds, not sure about android.
I don't know... how would I start by defining that I would also like to use the gentoo build chain and then select which packages should be built and added then add some custom contents and finally build the whole thing either iso, img, qcow2 and so on.
I mean, how did you start? Did you just take the chromiumos manifest and did it trial and error style?