The CAN transceiver is starting to look like a better option. If it can carry i2c, it should be able to carry UART just fine, and signal reflections aren't usually an issue at low speeds. I'm planning on using the mcp2561, which is the 1$ one I mentioned.
Although I have some spare wire pairs, I was initially planning for a single TX bus and a single RX bus. The CAN transceiver is differential, so either I dedicate another wire pair, or use a negotiated master/slave CAN bus. Not a high hill to get over, but it does add some additional software complexity.
Slower serial speed is not a problem for this application. I only need 10s of seconds of "simultaneity." (The concrete radiant floor weighs 90,000 lb. Its temperature isn't going to swing quickly unless the Russians nuke Cheyenne Mountain, which is nearby.)
The big attraction is the EMI reduction (slope control on the transceiver waveform) and noise resistance.