Hm... it looks like xbindkeys is more complicated than I thought. Probably
not a good choice, then. Apparently it requires manually editing a config
file, which isn't helpful since you can just do that for Openbox anyway.
Here's what you really want:
https://github.com/nsf/obkey
There's no Trisquel package, but it seems to be libre, and it's just Python
so source code installation shouldn't be a problem. So download this:
https://github.com/nsf/obkey/archive/v1.0.tar.gz
Extract to your Downloads folder. Then open a LXTerminal, and enter the
following commands:
cd ~/Downloads/obkey-1.0
sudo python setup.py install
After you do this, you should be able to run obkey by typing the "obkey"
command either in a terminal or in the Run prompt. That should give you a
reasonable graphical interface to adjust Openbox's keyboard shortcuts in.
Let me know how it goes and if you run into any difficulties! :)