So, job accomplished without removing anything else. Items needed: a couple of magnetic led flashlights, a 1/2" x 6" socket drive extension, an offset 13mm 12 point wrench, a 13 mm 6 point 3/8" drive socket and 2 ratchets, one 1/2", one 3/8". a shop crawler so I could lay on my back and slide around. I have a rear radiator so this is pretty much only accesible from under the engine.
I was able to insert the 1/2" drive extension in the slot in the adjustment plate without removing the serpentine belt. I used some pb blaster in advance on the pivot and adjustment bolts. I used the 13 mm socket to loosen the pivot bolt and the 13mm offset wrench to loosen the adjustment slot bolt. I then used the the 1/2" ratchet to tension the belt whilst simultaneously tightening the adjustment slot bolt. After that I retightened the pivot bolt.