You can write a script (to toggle a particular tag on and off in selected lines), and attach that script to a keystroke with something like Keyboard Maestro or Fastscripts.
I’m personally out of town at the moment, without a macOS machine to hand - but if no-one else has sketched a script for you by then, I should be able to take a look in the latter half of next week.
In the meanwhile, it may be worth looking at the tags section of the extensions wiki:
https://support.hogbaysoftware.com/t/taskpaper-extensions-wiki/