Instructions on how to get Gtk3 indicators in a Gtk2 Xfce-Panel

Prepare and install dev packages:
sudo apt-get build-dep xfce4-panel libxfce4ui xfce4-indicator-plugin
sudo apt-get install git xfce4-dev-tools gtk-doc-tools libindicator3-dev libido3-0.1-dev

Get xfce panel from git:

git clone git://git.xfce.org/xfce/xfce4-panel
cd xfce4-panel
git checkout andrzejr/wrapper3
./autogen.sh --enable-gtk3
# (above you can add --prefix=/usr if you're not afraid of reinstalling your xfce4-panel with "sudo apt-get install xfce4-panel --reinstall")
make
sudo make install
sudo ldconfig # (to update libraries)
cd ..

Get libxfce4ui from git:

git clone git://git.xfce.org/xfce/libxfce4ui
cd libxfce4ui
./autogen.sh --enable-gtk3
make
sudo make install
sudo ldconfig
cd ..

Get indicator-plugin from git:

git clone git://git.xfce.org/panel-plugins/xfce4-indicator-plugin
cd xfce4-indicator-plugin
./autogen.sh
# (if you used --prefix=/usr before, use it again here)
make
sudo make install
cd ..

Finally install some gtk3 indicators:

sudo apt-get install indicator-power indicator-bluetooth indicator-datetime indicator-keyboard indicator-printers indicator-sound indicator-sync indicator-messages
sudo apt-get purge indicator-sound-gtk2 indicator-application-gtk2

Run it:
Run xfce4-panel --restart and your panel might look half-empty. Be sure to either backup your config (~/.config/xfce4/..) or be ready to recreate your panel by hand. The first time you run it you might get crashes because gtk2 indicator backends are still running. You can ignore this.