It seems to work, more or less.
Accuracy isn't very good. 
I know the 1.1V is only a "nominal" but it reads very low on the chip I'm using. Still, it's a free feature so we mustn't complain.
I made the jar display the voltage to two decimal places. It outputs each digit as a series of pulses with a delay in between digits. A zero is a long pulse.
Here's one I made last May - it's been running 24/7 since then on the same button cell. and it's still going strong(!)
There's a magnet in the butterfly "key" to switch it on/off via. a hidden reed switch. It also has a mercury switch inside so you can shake the jar to make them more active (they slow down over time to save battery).
We're making some more in a workshop next week so I was just revising the software. I replaced the mercury switch with capacitive touch sensing (touch the lid to make them more active) and now it has voltage display as well! (turn it off and hold the key in place for a few seconds extra to see the voltage).