I was about to question the shelf life of the Mx-RTC, both uninstalled and installed when I read this thread. I would be very happy with a replaceable battery on the side. Many people have these already with a a routine to annually (here in the USA smoke and CO detectors get replaced when we set clocks from daylight savings time in the fall), I also have a Automatic transfer switch for a standby generator that has a 9 volt battery that gets replaced.
It would seem that 2 piggybacked DIP sockets with some bent pins, a couple of diodes, a little wire, a battery holder, and maybe another component or 2 would do the job.
Can TRI-PLC produce such an item? The current situation seems less than desirable for long term projects.