Trying to convert battery powered sensors to hard-wired sensors. Code / safety concerns?
Low voltage wiring to devices that draw less than 55 watts has greatly relaxed rules, and is covered under a different section of (US) National Electrical Code  than mains wiring.

You can see the effect of those rules in how thermostat and doorbell wiring is done -- both of which are subject to Code.  

Here, the very fact they can get 3 months runtime on two AAAs implies you have exceptionally low draw devices (in the microamps) that they are not using any more power than you might find on telephone or Ethernet cables (disregarding PoE).  So telephone style wiring is appropriate.  

Of course, telephones have current limiting resistors at the far end of the circuit, even if you're right next to the central office, the telco won't give you enough current for a short to start a fire.  

You should do the same: have current limiting resistors on the "head end" (power supply end) to remove the possibility of significant current flowing, e.g. A resistor calibrated so the devices normally receive sufficient current, but if the wires are dead shorted, only a few milliamps flows - would be about right.  So you might consider a 3V supply, skip the buck converter, and put a rightsize resistor on the head-end. 

E=IR

E=3V, I=10ma (30mw not likely to ignite drywall paper), R=300 ohm.  Assuming your device normally draws &lt;&lt;1ma, it will see &lt;&lt;0.3V drop, you'd have to give it some trial and error to make sure the device isn't browning out at times of key activity such as transmitting, to find the right value of R.