* Is it required to install a complete build system just to compile an X server?
Installing OE is enough, since this build system also builds the toolchain and the necessary dependencies for the X server.
* Does OE run on Mac OS X for cross-compilation?
I don't have a Mac, but I've been told that someone got it working. Check out the Wiki.
* Is it possible to use OE to compile for the Sharp ROM (glibc) or only for OZ? I can't force my users to flash a kernel just to install an X server.
That may work. We have glibc-2.2.5 in OE, although you would need to tell OE to use your prebuilt 2.95.3 toolchain. Since no one of the OE core team is interested in the SharpROM, it would be nice to have someone looking if that works (and if not, supply some patches )