I wrote this script which I can call from a keyboard shortcut using FastScripts or add as a script in the OmniFocus tool bar.
You can get it from my github repo.
osx_scripts_folder/Toggle Tag.scpt at master · bgebhardt/osx_scripts_folder
Hope this helps someone else. I would also love if the OmniFocus team could bind tags to certain key combinations. I would use that quite heavily. But for now I have this script. Automation and AppleScript for the win!