Referring to this post, you can exploit a pcmanfm setting whereby you can set the right click to open the openbox menu (instead of the file-manager menu).
Preferences > LXQt settings > Desktop > Advanced
Show menus provided by window managers when desktop is clicked
This menu is found at
usr/share/lubuntu/openbox/menu.xml
(but the items don’t work because lxsession-default / LXDE is no longer the default session manager). However the openbox menu mechanism still functions.)
Make a backup copy and edit it to what you like.
You will need administrator privileges or you can change the file ownership.
As an example, I changed lxsession-default calculator with kcalc to open the calculator.
To restart openbox, use
$ openbox --restart
(if you prefer not to change the system wide menu.xml, there is a more long winded way to do this using a local file. PM me if you need this).