I had no problem accessing the back bolt head on this tensioner pulley fastener by just using the open end of my trusty Craftsman combination wrench:
Just make the open-ended wrench is fully seated on the bolt head before applying full torque to remove/reinstall this extremely large (24mm?), special nut. By the way if you don't have a (not common) 24mm socket, I found a 15/16" socket fit nice and snugly, and worked very well.
If you end up removing the nut completely, I'd recommend scraping away any old threadlock goober you find, so you can reassemble (and bring up to full torque) with fresh, clean metal surfaces. You want the full clamp load to travel through the metal surfaces, not through old dried threadlocker.
If one ever wants to remove the fastener in the future without damaging things in the process, I'm not sure that the red (permanent strength) threadlocker is a good idea. Blue threadlocker I think is more appropriate for this high-vibration fastener.