I don't understand why the necklaced octo is not standard. The PADI instructor guide clearly shows that the octo placement should be inside of the triangle as shown below. I think a necklaced octo would fall into that same area - don't you? Ironically most divers clip their octopus on the right side of the BCD, which would fall outside of the shown triangle. Even yet most fall out of the octo keeper and float behind the diver and or drag on the bottom. The necklace is the way to go.
FWIW, the regulator in the necklace isn't the "octo", as it's not what you'll be donating. You'll donate the one you're breathing from, and then go to the secondary on your necklace.