Think about it: you are creating a graphical application that launches a
graphical windowing system. Therefore, you have to already have a graphical
windowing system of some form running. At last count, that means that you
are running either Windows, Mac OS X, OS/2, or maybe BeOS. You certainly
can't be running on any platform that uses X11 as the graphical windowing
system, because you would have to have X11 running in order to launch X11
via your launcher.
Now, the Mac OS X folks have lots of neat ways that their server is already
tied into the Mac OS X-specific startup methods and such. Supporting BeOS
would be a silly academic waste of effort. OS/2 is similar, but good luck
finding a cross-platform toolkit that includes OS/2 support.
So, what are your real targets here? Have I missed some operating system
that also needs an X launcher utility?