Emacs does understand X resources, so this is a working solution for X in general, not only in Xfce. (This is however not true for any program; for example, xfce4-terminal cannot be controlled this way.) The X resources can be viewed by xrdb -query.
To achieve what you want, I have put into /etc/X11/Xresources-site (/etc/X11/Xresources is also OK, though can be overwritten by your distro):
Emacs.fullscreen: maximized
This would affect also remote X clients which are Emacs (e.g., emacs started on a remote host via ssh).
/etc/X11/Xresources-site and /etc/X11/Xresources (and probably ~/.Xresources and ~/.Xdefaults) are usually read at the start of your X session; to affect your current X resources immediately, run something like xrdb -merge /etc/X11/Xresources-site.
Actually, in my case, /etc/X11/Xresources-site is being read thanks to a line in /etc/X11/Xresources (which is read by the start scripts):
#include "/etc/X11/Xresources-site"
so /etc/X11/Xresources is read for sure.
There are also some files with the same syntax which are read each time an X program like emacs starts. In my case, they are: ~/.Xdefaults-MY_HOST_NAME, /etc/X11/app-defaults/Emacs (only for emacs-athena, not for emacs-gtk3), /usr/share/X11/app-defaults/Emacs etc. (But I like the idea of loaded X resources more -- shown with xrdb -query; so that remote X clients read the same X resources.)
Other X resources which Emacs understands are described at https://www.gnu.org/software/emacs/manual/html_node/emacs/Table-of-Resources.html#Table-of-Resources.