Yes, you could trip the system to release the CO2 which should shut down the engines. Obviously this may not be the best way to do so - if your engines have air doors (not all do), that's the best way to shut them down if the normal shut down doesn't work.
If you have an oem Hatt CO2 system and IF it's set up the same way as it is on our 53MY, when the system is activated CO2 is discharged into both eng rooms and the genny room. At the same time, the engines are shut down by the normal shut-down solenoid through a separate elec switch attached to the CO2 lines and activated by the CO2 pressure. So the engine shut down solenoid is activated an instant before the CO2 actually starts spewing from the nozzles. Obviously, one of two things will actually stop the engines - the solenoid or, if it didn't work, the CO2.
The CO2 system is activated (either by the sensors or manually) by a spring-loaded firing pin that, when tripped, releases the CO2. Although I haven't seen it happen, the PO of our 53MY had the system activated by an errant grand-kid who pulled the manual release. He ( the PO) assured me that if the system is fired, the noise will NOT go unnoticed by you or anyone within several hundred yards of the boat!
I have taken the control head, which contains the firing mechanism off the system and tested it to be sure it works properly. I don't think an "accidental" release is anything to be at all concerned about. By "Accidental," in this case, I mean where the system itself malfunctioned and tripped with no outside aid. Obviously, you always have the possibility that someone could pull the manual release. Essentially, it is exactly the same as the firing mechanism on a bolt-action rifle. It is quite safe until somebody pulls the trigger - no, it doesn't have a hair trigger!!
After recharging the system and to avoid a repeat of the "accidental" discharge (actually, it wasn't an "accident" at all, the kid PULLED the release which then performed exactly as it should have), the PO disabled the system by unscrewing the control head from the bottle sufficiently so that the firing pin couldn't hit the release plug. He would do this when the kids were aboard and retighten it when they weren't. As you can imagine and since his memory was no better than mine, he had forgotton he had done this and the system, though fully charged/weighed/certified, didn't work when I bought the boat. I didn't discover this for several months after buying it and only then because someone on this site happened to post about the system and how to check it. So I thought "Hey, I should take a look at that!" Good thing I did.
So it's a good idea to thoroughly check the system to be sure it really does work. A full CO2 bottle means nothing if the sensors, sensor lines, or firing mechanism aren't functioning properly. You can do this by removing the control head from the bottle and pulling the manual release. You will hear the firing pin release with a loud snap. After resetting it, you can do the same with each sensor using a hair dryer to heat it - in 15 sec or so, the firing pin should be released. Repeat with every sensor.
Note that a surveyor, or whoever will NOT check to see if the system works. They will only look at the tag on the bottle to see when it was last weighed.
Again, I think an accidental firing is extremely unlikely and I would never worry about it at all. Heck, if I was in the engine room when it was activated, I'd probably die of heart failure from the noise long before the CO2 would have a chance to do anything!
One other note - re air doors. Don't assume they will work unless you have checked them AT IDLE - NEVER CHECK THEM AT ANY SPEED ABOVE IDLE. DD says they should be checked annually. One of mine would release properly but would not shut down the engine. I had to remove the air horn and "adjust" (bend!) the door.
Basically - don't assume ANYTHING on the boat works properly until YOU have seen it work. DON't rely on a surveyor to tell you it works. I could make a fair-sized list of IMPORTANT items that were "OK" on surveys but didn't actually work when they needed to. Bill said, "Trust, but verify!" To me that's gov speak for "don't trust anybody," which is sound advice!
