Okay, so it's happened to someone at least once...which pretty much nullifies any argument that "this is a non-issue."
Thank you for sharing.
what
@claymore experienced was operator error from a poor equipment configuration choice. You shouldn't solve that problem with a new piece of equipment, you should change the configuration to a more appropriate solution. In that case, run the drysuit hose like it is supposed to be run. I have hundreds of drysuit dives and have never had a drysuit hose anywhere near my tank valve, and I suspect that Claymore hasn't had any near misses since he changed his configuration.
The whole point of that circles back to an equipment solution to a skills and/or configuration problem. In this case you have two skills problems, and one potential configuration problem. I'm an engineer, so I'm with you on wanting to make sure everything is as good as it can be, but in this case you have to step back and look at the problem. In this case it is a valve turning off, multiple full rotations before closure, where the diver is oblivious that his tank valve was in contact with something, unable to reach his valve, and his buddy was unable to reach it.
So the skills problems are the diver is oblivious *not so easy to fix*, and doesn't know how to ask his buddy to check his tank valve *easy to fix*. The potential configuration problem is he can't reach his valve which is easily remedied by moving the bcd down on the tank.
Now, the big con to your potential solution is that it has serious ramifications for non-recreational divers in the form of the inability to feather valves which
@The Chairman mentioned as a reason that you need to reach them. Your essentially negligible problem isn't actually a failure, just a nuissance, but a relatively common issue is a freeflowing regulator. If you had a valve that required two motions to unlock and turn, it would be very difficult to shut down the gas. The ability to quickly shut down that tank is HUGE in technical diving because every second longer it takes to turn off, the more gas is wasted. In a cave, that could be the difference in making it out or drowning.
Minor con is that it takes something that is idiotically simple and makes it unnecessarily complicated. You said yourself above that you want to remove points of failure. This type of valve adds several failure points that do not exist in a standard valve.
So, do the valves exist? yes, they exist. They are standard on SCBA tanks for firefighters. They could be very easily adapted for the scuba market, but the one potential issue they solve creates more with much more severe consequences.