Well I managed to do some experimenting with inverted tanks.
I initially started with my Luxfer Al 80's but found that, in order to reach the valves, I had to mount them quite high up on my back. Besides having big squarish butts sticking up near my ears I also figured that they would ride far too top heavy to really be practical. They were also quite heavy to doff and don and I was worried about banging the valves (I was alone) so, for a variety of reasons I gave up working with them.
The first thing I learned is that having a valve protector makes all the difference in working with inverted doubles. With them set up would be fairly straight forward; without them it was both heavy and worrisome.
I switched to a pair of decommisioned AL tanks that I have which are the same dimensionally as ST72's. Much better. You will notice they don't actually have k valves but I was able to position my hands where there ought to be valve knobs by using a mirror and feeling. Here's a shot of the tanks mounted on a backplate showing some hose routing up behind it (the backplate). With banded doubles the hoses would route nicely between the bands:
I tried to imagine manipulating an isolator valve and found that it would present some problems inverted. I could reach the valve somewhat if it were pointed straight at my back but it would also dig into my back (well ok, A$$) as well. If I rotated it downward to avoid gouging my back I found I could not reach far enough to manipulate it very well at all. In the end I suspect a isolator valve extender (AKA slobknob) would be needed. How this would route to be both accessible and workable I don't know.
Here I felt a bit like those guys on mythbusters. You cannot just reach between your legs to manipulate the isolation valve. The more you try; the further away it goes.
But reaching the left/right valves was far easier inverted.
No custom reg hoses are needed. Here I am using a 40" hose routed up behind the backplate.
And a 5' longhose routed in the traditional way.
Donating poses no problems
In the first photo you could see an SPG routed up behind the backplate. With a 36" hose it clipped into my chest D ring over the shoulder (ghosts of another thread). Unfortunately I didn't take a picture. Here I tucked it into the cam band and clipped it off on the hip. With some thought a cleaner routing solution using standard hoses could be found.
Drysuit hose, no probem (and yes I know it's trapped under my waistbelt).
Now I learn something new.
First, a standard 36" LP BC hose is available which could route up behind the backplate to be used with a wing in the conventional way... but then I remembered several threads wherein sidemount divers said they switched the bottom dump valve for the inflator hose to accomidate their hose routing. The last picture shows my attempt at this and the obvious results. Now I have to start a new thread somewhere to ask just how they do it?
I also clipped off a stage bottle at one point (again no photo) and it didn't really present a big problem as far as accessing the right/left valves (it was the same as finding the SPG clipped off at the hip). That was only one stage though (Al80) beyond that I can't comment.
In the end I learned some things about inverted tanks.
A valve protector is needed to even work on them. I thought of several ideas along the way and such a devise wouldn't pose much of a problem for anyone with welding/fabrication skills but only a crazy person would dive without one. During my little "experiment" I couldn't help but marvel at the fact that those Cousteau packs really were both practical and cosmetic. All the routing was set out and everything was well protected at the same time as presenting a minimal profile for entanglements.
Diving with St 72's or other small tanks would be ok but larger tanks would, at some point, begin to present balance and profile problems.
With some forethought, hose routing doesn't present much of a issue and standard length hoses would work.
Using an isolation manifold would be problematic without an extender (the efficacy of which I can't comment on as I have never used one). Diving with independant twins would (all other points being equal) allow easier valve access to the left/right valves.
I personally wouldn't dive the system except as a novelty as (like Sas stated earlier) it doesn't really solve any problems for me. If I had compromised shoulders that couldn't be corrected I would give BM or SM independants twins a long look first.
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