configure ToggleShowDesktop in Openbox to only act on specified layer

Openbox will give the functionality you are looking for to windows whose _OB_APP_TYPE property is set to "desktop" (You can use obxprop to check the properties of a window). So we need to set the _OB_APP_TYPE for your terminator window to "desktop" so that this will happen.

Reading through the openbox source code, in client.c I could see that _OB_APP_TYPE is inherited from _NET_WM_WINDOW_TYPE. So I looked for an application that could change _NET_WM_WINDOW_TYPE. The application I found is called devilspie.

First, install it:

sudo apt-get install devilspie

Next, we'll create the folder for devilspie configuration files:

mkdir ~/.devilspie

Now we will add a config file that tells devilspie to look for applications named terminator and set the window type of them to desktop. Put the following into ~/.devilspie/terminator.ds:

(if
(is (application_name) "terminator")
(begin
(wintype "desktop")
)
)

Now, if you run devilspie (or devilspie -a to affect existing windows instead of just newly created ones) you'll notice if you use obxprop on terminator that _NET_WM_WINDOW_TYPE has changed, but _OB_APP_TYPE has not yet. Let's use xdotool to unmap and remap the window (in X11 terminology this means we will stop drawing and begin drawing the window), which should force openbox to notice the value.

Install xdotool:

sudo apt-get install xdotool

Then we run the following script to find the terminator window, unmap it, wait until it has been unmapped, then remap it. We also resize the window because when it was unmapped and remapped it lost its maximization:

xdotool search --class "terminator" windowunmap --sync windowmap windowsize %1 1024 768

Replace 1024 768 with your resolution. Also note that newer versions of xdotool than those found in Debian's apt repo support using 100% 100% instead of a static resolution. You can find .deb packages for newer versions of xdotool at xdotool's googlecode page.

The terminator window will immediately become undecorated, and if you now use obxprop, you will see that both values have been set properly. If you now press the key you bound ToggleShowDesktop to a few times, the terminator window will always stay active.

To make this persistent, you'll want to run these commands on login. The LXDE Wiki Page for LXSession shows several different ways to do this (global, per-user, per-profile, etc). Assuming you want these settings just for your user and under the LXDE profile (the default), you'll want to edit ~/.config/lxsession/LXDE/autostart and add the commands we ran, keeping in mind that the xdotool command needs to be run after terminator's window has been rendered, so do something like (sleep 1s & xdotool ...). You could also set these commands up as keybinds in openbox if you wish.

I came up with the following script which looks like it solves your problems.

#!/bin/sh

make_wid_desktop_app() {
    # set the type of the window to 
    xprop -id $1 -f _NET_WM_WINDOW_TYPE 32a -set _NET_WM_WINDOW_TYPE _NET_WM_WINDOW_TYPE_DESKTOP
    xprop -id $1 -f _MOTIF_WM_HINTS 32c -set _MOTIF_WM_HINTS 0
    xprop -id $1 -remove _MOTIF_WM_HINTS
}

get_wids() {
    xdotool search --class $1
}

for id in $(get_wids $1); do
    make_wid_desktop_app $id;
done

You can use it like this.

desktop_terminal terminator-wallpaper

It searches for all windows that have the class terminator-wallpaper with xdotools and change there _NET_WM_WINDOW_TYPE to _NET_WM_WINDOW_TYPE_DESKTOP with xprop.

Since Openbox does not check if this type was changed after a window was created we need to trick Openbox into doing this. This gets done by setting the _MOTIF_WM_HINTS and removing them.

The same thing can be achieved with devilspie when you add

(if
   (matches (application_name) "terminator-wallpaper")
   (begin
       (wintype "desktop")
       (undecorate)
   )
)

since setting the window to (undecorate) is also using the _MOTIF_WM_HINTS trick.