Once again, I should have Googled first and I would have found my
answer. The Firex manual for one of their interconnected smoke
detectors says...


This smoke alarm should be installed in accordance with the NFPA's Standard 72 (National Fire Protection Association,
Batterymarch Park, Quincy, MA 02269) and any local codes that apply.

When interconnecting, use #18 AWG minimum solid or stranded wire. Maximum wire length between any two is 1,500
feet for #18 AWG or 4,000 feet for #14 AWG (20 OHMS loop resistance).

DO NOT connect to any other type or model of heat or smoke alarm. Connect smoke or heat alarms to a single AC
branch circuit. If local codes do not permit, be sure the neutral wire is common to both phases.


So, I think that explains why I haven't found a commercially available
relay module to do what I was looking for.

Does anybody have a good idea for a passive way to monitor if the smoke
detector is going off? I guess I'm thinking something like a sound
pressure level sensor or something like that. Obviously it wouldn't be
foolproof. Any other ideas?