Finally got around to installing XFree 4.2, and noticed an oddity: XDarwin is placed in /usr/X11R6/../../Applications, which usually translates into just /Applications. Not so if you've symlinked X11R6 to somewhere else, however. In my case, /usr/X11R6 is a symlink to /Volumes/Procyon/usr2/X11R6, which means that XDarwin showed up in /Volumes/Procyon/Applications, where Sherlock eventually found it. More of a note than a bug report, really, if someone else run across the same thing.

Both Xinstall.sh and the InstallAnywhere Installer work this way.