Peter, sorry I missed this:
For a wired-or/and you can use logic gates, but also 12 diodes with a pullup resistor.
The collision detection could be a problem with 12 units. What if they resend with the same timing ?
It is better if the central unit polls the units one by one. The central unit TX is connected to all the RX and the central sends "0t" to request the temperature of unit 0. When you add a timeout, you can detect a missing unit.
Repeated collisions can be avoided if you use a randomized time delay for the response, but I like your polled idea with the timeout. One big advantage would be that all the data points could be input in the span of just a second or two.