Time to pick this apart.
Virtually anyone can do a Discover Scuba Diving session with a few minutes of training demonstrates that open circuit is incredibly simple to use safely. CCR diving is so fundamentally different from open circuit diving that it's effectively a "legal" requirement to take a 5 day training course in recreational CCR diving that has course standards well above and beyond that of AOW + Nitrox. There's 5 days of skills taught which are CCR only. Post course you must practice and keep those skills up.
In other words basic OC diving is simple whereas 'basic' CCR is most definitely not.
The problem with DiveMasters is most are not masters at all. They have very limited training and experience for the specific task of shepherding novice OC divers. To be fair, there's a small percentage of DMs who actually go beyond the limited PADI et al fold. But the vast majority of DMs have absolutely no knowledge of rebreathers nor technical diving.
Whilst your point is valid: it is obviously a "good thing" to have that final safety check, the problem is what "one" check just before you jump would be useful -- given that CCRs have so many novel ways of killing you.
Asking "is your oxygen tank on" superficially seems useful but is really just an (important / vital) OC check for novices. In CCR it just isn't that simple. Whilst people have been killed as their oxygen cylinder was turned off, many many more have been killed because they failed to monitor their PPO2 and ignored alarms flashing in their face -- the Head Up Display and/or Nerd.
Short of sitting the DiveMaster down and teaching them the pre-dive process and give them a laminated checklist, it would far better if we got them to ask a sensible question that gets to the bottom of the problem...
"Inflate your wing and tell me your PPO2"
The answer will be a hiss and demonstrating a PPO2 something above 0.7. Ideally the diver should point to their dive controller to show the (normally 3) cell readings within a gnats todger of each other.
That simple question means that the diver's demonstrated that:
- The oxygen is turned on and is being held above the surface setpoint (below which the solenoid, if fitted, injects)
- The diluent is turned on so they won't sink and can breathe on the way down
- The electronics are turned on and working; the cells are more or less the same (they'd show flashing red or amber in some controllers if wrong)
- The diver is awake and thinking
- The diver can can see their controller (not turned over on the wrist, tucked into some waistband, etc.)
- That the pre-dive checks have probably been done