Ok so a quick update:

I stumbled upon the latest generation of TI sensor tags, based off the CC 2650. The tag has a temp, light intensity, and humidity sensors on it. It also has a 9 axis IMU but I won't be using it. It claims to get over a year of battery life at 1 Hz update rate off of the a CR2032 coin cell... Which helps in the number 2 wife requirement, no visible wires. :) So I'm thinking of picking up several of them to act as my sensing nodes. After some looking around for a USB-Zigbee dongle with actual support, I came to the conclusion I would probably be stuck with Bluetooth as my communication method between the nodes and the controller.

For the wall mounted control unit in my living room, I've got 4 wires coming up from the basement currently switching 24VAC power to controller the furnace, fan, and AC compressor. My thought is to temporarily re-purpose those wires to provide DC power to the controller and establish a bi-directional serial link to a relay slave. The relay slave would be located at the furnace and would switch the 24VAC. It would also send relay status information back to the controller. My intent was to monitor the relay status directly by using one of the poles on a DPDT relay, that way I know fairly certainly if a contact refuses to make or break. I'm not too worried about contact welding as it's only 24VAC, current limited by the transformer. Lastly I would put a buzzer on the relay slave to sound loud and proud if communication with the control node is broken for more than 30 seconds.