I'm not recommending it in any way; but... There are two ways to do necklaces. You can actually zip tie the bungie onto the mouthpiece, or you can have a bungie loop that the mouthpiece inserts into... thus making it possible to pull the reg from the necklace. I don't like that method because it is difficult to restow and there is a chance that it comes loose during a dive; but it's not really worse then having the octo anywhere else, it puts it in the "triangle", and it's a step in the right direction.
When I first went to a BP/Wing, I tried a number of different things. I also dove a pony bottle. The mounting system I had to mount it upside down on my back [so I could reach the valve] flopped too much when I went to the BP because of the location of the straps. The options I had left was to mount right side up [and sacrifice reaching the valve], buy a new mounting system to mount it to the tank [making tank switches more annoying], or stage rig it. So I chose the latter.
Once I stage rigged it, I didn't like the valve and reg on the same d-ring as my octo scumball. I had been drawing the octo across my body like that to keep the hose a little tighter to my body. By putting it on my right d-ring, the hose looped out too far to my liking. The two remaining options I had was shorten the hose, or lengthen it... to 7 feet... and use it primarily.
What's the point I'm trying to make? As I fiddled with the things I didn't like in my setup, the reasons for some of the principles that are out there started coming much more clear. I found myself often trying to come up with a solution where there was already a concept out there that would work; and I decided that it was as good, or better, than any solution I came up with myself.
So you may find a place to put your octo; but it might have flaws that going another direction [in this case, long hose & bungied backup] may start to make sense as a viable option.