P.S. I have no idea how the SSJ would work with very negative steel doubles, though.
Very well, especially if you dive dry. I have done an old SP Master jacket with twin steel 10Ls (can't remember what that is is US measures) and it was nice to get a bit of weight off my belt, and the jacket remained confortable, and the cylinders stayed still on my back.
I really don't understand why people are so fixated on the BP/W thing. As DaleC points out, we started many years ago with simple harness attached to our cylinders, then had "backpacks", then backpacks with a horse collar, then BCDs. Most BCDs, except modern lightweight travel ones, still have a BP in them.
My SP jacket has a packplate as part of it. The only difference with a BP/W is where the bladder is located, it's a wing held between the two halves on the BP, keeping the inflation between the wearers back and the cylinders. In my SP jacket its a jacket held between the two halves of the BP, with the air distributed around the upper torso.
Now, I dive in a drysuit, and use this for all my U/W buoyancy control. The only time I use the BCD is on the surface, and with nearly empty steel tanks on my back there is not much pull backwards at the surface from these tanks, and I feel, very little need to place the bladder closer to the tanks, however, for some people this is a problem, and they move the bladder. Fine.
As beanojones points out, you can always add a wing there too. I have done this too (photo on my profile), simply because under certain conditions my SP Master jacket doesn't have enough lift incase I flood my drysuit, but it is hardly ever used.
Each to their own I suppose.
Jon