I don't see need for building of separate X11 libraries in prefix, because system X11 could be used, and API and ABI of these libraries are pretty stable. You may require installation of latest XQuartz if system ones are too old
I'm not sure if I understand your request correctly. The time that we used as much from the host system as possible is gone. We got way too many problems on that route, so we provide everything ourselves.
I'm talking about virtual packages for libraries like libX11, libXau, etc
Sorry, we don't use the host system libs on purpose unless there is no choice (libc). You may flesh out this idea on the gentoo-alt mailing list if desired, and I encourage further discussion, but bugzilla is not the place. Closing bug.