The code to show something only in XFCE follows the same idea:
Unfortunately, as far as I know, the way XFCE stores it's .desktop files is the same as the GNOME stores them, whereas KDE stores them in a separate folder (within /usr/share/applications). This is why the separation of KDE/GNOME, as described in the post you link to, is much easier to do en-mass.
All the files are stored in /usr/share/applications. The format of them is very simple, and you can copy any one of them to /home/you/.local/share/applications and it will override that menu entry for that user so you can more easily edit them without needing root privileges.
There is no simple way to separate XFCE apps from GNOME, especially since it's often a matter of opinion what you want to see. This also goes for KDE, since as long as you have the KDE libraries Dolphin or Konqueror will run in GNOME. I suppose if you're intent on using the cmd line you could come up with some grep/sed junk to selectively find and replace text inside certain .desktop files, but I thnk the easiest way would be to copy the files to ~/.local/share/applications so they don't get overwritten by updates and edit them by hand. Unless you've got a gazillion things installed it shouldn't be too difficult.
The menu works under the freedektop.org specification, which you can learn more about here: