DA-Aquamaster speaks more truth than he knows.
1) Head strap for regulator... It's been done and it wasn't real good.
Yes in the 70's and before, most regulators came with a head strap (really more of a neck-strap). And indeed it was insufficient to protect unconcious divers for reasons given by DA-Aquamaster above (inadequate mouth seal).
Now, here is why this practice died off: In those days only professionals or instructors had secondary air sources. Average Joe diver and his buddy each had only a single second stage to breathe from; no secondaries. In an AO emergency, you shared your buddy's single second stage by alternating taking breaths from it (two breaths at a time) and and passing it back and forth while making your ascent. This was called buddy-breating and is currently considered a dangerous to practice. (It should be obvous why.)
Now, if your buddy's second stage was attached to his head with a strap, you couldn't pass the reg easily, Even more important, when you were gasping and shuddering frok lack of air you couldn't just swim up to buddy and grab the reg out of his mouth to snatch a lifesaving breath if it was stgrapped to his head. You would probably end up pulling it out of his mouth but be unable to get it to your mouth leaving both of you choking on water.
So, the head strap was considered a hazard. Instructors and dive shop operators counseled divers buying new regulators to remove the strap that came with it and throw it away. Come the advent of "octopus rigs" with secondaries, in the mid 70's and the strap was still considered a hazard because (and this is still the case), the out-of-air-buddy who is gagging after inhaling seawater is much more likely to come up to you and snatch your primary out of your mouth than to look around trying to find you secondary or to politely sign "out-of-air" and wait for you to find and pass the secondary. Thus even today, the headstrap would be considered a hazard in an AO emergency and so it is not used.
2) Regs with alarms that announce low air... Been done and really not a bad idea:
Prior to the mid-70's a SPGwas a luxury or a toy. Some instructors or guides or deco divers had them but average Joe did not. Instead what he had was an alarm that would tell him when his tank pressure dropped below 600. Since we didn't do safety stops then outside of deco diving and since there were no BCD's to inflate, 600 reserve for a non-deco, open water dive was pretty much adequate. You only needed enought to make your ascent.
The alarm provide was more compelling than any audio or visual alarm and could not be ignored because when the tank pressure dropped below 600 the alarm signal was to cut off the diver's air. The diver was then forced to pull a lever to release the 600 psi reserve in order to continue to breathe.
Now air was not cut-off suddenly but gradually so at first you might not notice and then a minute later you'd say "Wha? Why is it getting hard to breathe?" and then you'd say "Oh yeah! I must be down to my reserve." and you'd pull the reserve lever, note the time and make your ascent. This alarm was located in the tank valve rather than the regulator and it was called a "J-Valve" and you can still buy them on Ebay today. They are very reliable but now that everyone uses an SPG, a J-Valve is just one more thing that might malfunction so it has gone out of vogue.
I wouldn't mind seeing them come back with a user-settable reserve pressure so you could dial in 1500 for that offshore drift dive or 500 for that shallow practice dive 20 yards offshore in the quarry.
