Take the following commands and bind them to separate Super-key combinations:

Commands for Xfce 4.12 through Xfce 4.14:

The following commands are now used to set the auto-hide properties:

xfconf-query -c xfce4-panel -p /panels/panel-0/autohide-behavior -s 0
xfconf-query -c xfce4-panel -p /panels/panel-0/autohide-behavior -s 1 
xfconf-query -c xfce4-panel -p /panels/panel-0/autohide-behavior -s 2

Commands for Xfce 4.10:

xfconf-query -c xfce4-panel -p /panels/panel-0/autohide -s false
xfconf-query -c xfce4-panel -p /panels/panel-0/autohide -s true

Update: 12/09/2018: As mentioned in the comments, the above commands are no longer supported.

Command Values:

The value '0' is used for 'never' auto-hide. Value '1' is for 'intelligently' auto-hiding the panel, and '2' is used to set the value to 'always' auto-hide.

Setting the key-bindings:

This can be done by either selecting, Applications Menu → Settings → Keyboard → Application Shortcuts, or by running xfce4-keyboard-settings from the command line(Ctrl+Alt+t).

I've bound the first command to Super+u (to 'unhide' the panel). For the second command, I am binding Super+h (to 'hide' the panel).

As you can see in the commands above, the behavior is only changed on a single panel(In this case, panel-0 is being modified).

You may notice a slight delay while the panel changes state.

More details on the xfconf-query command can be found in the Xfce documentation or from this thread in the Xfce forum.

Try this workaround:

Install xte and xdotool with sudo apt-get install xdotool xautomation

You can use xdotool to find the x,y coordinates of the mouse. Open a terminal type xdotool getmouselocation then move the mouse somewhere where it will activate the panel and hit enter. You will get some output like:

findclient: 62914741
findclient: 6291474  
x:1282 y:1079 screen:0 window:62914741  

What's important here is: x:1282 y:1079, which gives us the x,y coordinates of the mouse.

Now type xte 'mousemove 1282 1079' (replacing 1282 1079 with the coordinates you got earlier). That should move the mouse where you want.

You can use xbindkeys to bind this command to a key on the keyboard. I can add instructions upon request or you can set it in Xubuntu's keyboard settings.

Edit:

@JQuigley pointed out "This no longer works..." see comments.

As Kevin said, you can use xfconf, but I'd do it this way:

xfconf-query -c xfce4-panel -p /panels/panel-0/autohide -T

The -T toggles the true/false value. This way, you can use the same key to hide/unhide.

To get more options do:

xfconf-query --help

I wanted to make a comment to the answer from Kevin, since it's what got me on the right track. But I'm new here, not enough reputation..

I know I'm a couple of years late to the party, but just for anyone who might see this in the future: Here is a bash script that toggles autohide in the new system, simply save this in a text file somewhere, run chmod +x file/that/i/saved and set the script to a key in Settings -> Keyboard -> Shortcuts.

#!/bin/bash

cur=$(xfconf-query -c xfce4-panel -p /panels/panel-0/autohide-behavior)
if [[ cur -eq 0 ]]; then
  nxt=1
else
  nxt=0
fi
xfconf-query -c xfce4-panel -p /panels/panel-0/autohide-behavior -s $nxt

topisani's script above still works well in xfce 4.12. You can change the target panel by changing panel-0 in the script to panel-1 or panel-2 etc. Don't forget that there are two lines to change.

EDIT:

It's been pointed out in a comment that the autohide property has been changed to something else, and -T doesn't work with the new one :/

So, while I was fooling around with this, I made a little ruby script to toggle all panels at the same time. It seems to work.

#!/usr/bin/env ruby

#Toggle all panels' autohide property in xfce4
# fede s.
out = `xfconf-query -c xfce4-panel -l`.split
out.inject(Array.new) {|res, val|
    m= /panel-([0-9]+)/.match(val);                 #get the panel numbers
    if m then res.push(m[1]) end;                   #add only if it matches
    res}.sort.uniq.each do                          #filter duplicates
        | num |
            `xfconf-query -c xfce4-panel -p /panels/panel-#{num}/autohide -T` #here is the command that will repeat for every panel found
    end

It's probably not the best way to do it, and I don't really know much about ruby, so feel free to throw me in the fire! :P

If Someone want's to use it:

1. make sure you have ruby installed.

2. copy it to a file wherever you see fit (I have mine as ~/scripts/xfce/toogleautohide.rb).

3. Make it executable

   chmod +x path/to/your/script
   

I have tried both 1. moving cursor with xdotool mousemove_relative and 2. changing panel property hide=always/never with xfconf-query -c xfce4-panel. But the best behavior was with 3. changing x window state. Script example:

#!/bin/bash

#command will get several uids (if there are 2 panels then probably 2). Need to find out which one we need.
windowUids=$(wmctrl -Glx | awk '/[Xx]fce4\-[Pp]anel.*[Xx]fce4\-[Pp]anel/ {if ($3 !~ /-9999/) print $1}')

#i need the panel with y coordinate = 0, while other panel has other value
for uid in $windowUids
do 
  yCoord=$(xdotool getwindowgeometry $uid | grep Position | cut -d',' -f2 | cut -d' ' -f1)
  if [ $yCoord -eq 0 ]
  then 
    WINDOW_UID=$uid
    break
  fi
done

#if window is viewable then hide, otherwise show
if xwininfo -all -id $WINDOW_UID | grep IsViewable
then
  xdotool windowminimize --sync $WINDOW_UID
else
  xdotool windowmap --sync $WINDOW_UID
fi

Here i find out which panel to process by y coordinate, you may need your own check e.g. by x coordinate or by width/heigth (see what xdotool can get). Also do not forget to set "Dont reserve space on borders"=true in panel properties, or other windows will be changing their own sizes.