Take a look at Crossover, a commercial MacOS app that runs Windows XP apps along side MacOS apps. Crossover is based on X11's Wine and "bottles", it uses its own server and quartz-wm. You really, really want these to be self-contained because there are be situations when one would want to run Crossover and X11 at the same time. For example that would let me have Windows XP, X11 and MacOS apps open simultaneously side by side. Developers have a range of options how to do implement X11 in a user-space app, seems there should be some kind of standard. Maybe I am missing something.