I'm not sure what the quiescent current of the RTC chip is, and don't know how it will react. It can't require much or the button cell wouldn't last long.
However the point of the RTC on this board is only to deal with short network disruptions and the time will be restored as soon as a network is found, so I don't really see the point of putting that much effort into the RTC circuit.
But sure, it's your design. That was the point of this board: People should do what they want with it.
BTW, would you offer me back the board design and the current code, and I'll contact you off line about if you have any more tubes. I'd like to make one as well.