I can't speak to the regulatory side of this, other than to say I'd be very surprised if a custom-built circuit like you're describing would pass an inspection just for the fact that it's a non-UL/CSA approved device on a life safety circuit. Being in a private residence at least will relax the requirements compared to a commercial or multi-tenant building, but connecting non-listed circuits and devices is tough in any case.

Rather than using a custom circuit, there are interconnect relays on the market that are designed for doing precisely this type of thing. For example, Kidde has a SM120X module that is designed to interface to external strobes/sirens/etc, as well as accept activation input for the interconnect circuit (using pull stations and rate-of-rise heat detectors as examples).

You should be able to use one of these and hook up your flow sensor in place of the pull station in the diagram above.

To me, it seems there is a low risk of impact:

• This device is UL-listed
• It's a manufacturer device, designed to interconnect with the manufacturer's own products
• In this case, the signal between sprinkler and fire alarm is really a secondary system: if there is smoke, whether the sprinklers have activated or not, a detector should be activated and signal the other alarms anyway. In other words, if this device/connection fails in anyway, it is not critical to safety.

I'd use a round plastic cut-in box even though you have access behind. You'll find this much easier in the long run.

Remember, smokes do not have ground connections, so just splice the grounds with a wire nut or other approved method and fold into the back of the box.

Along the lines of this: