I think you'll be OK... Go ahead and give it a try! If there are problems you'll know rather quickly.
You might want to calculate & compare the self-discharge/shelf life to the drain at 30M just to see what practical effect it will have.
Then of course, there's the question of how long you'll need to run on the back-up battery when you loose power.
I have read wet air ...
Humidity isn't a problem until it gets to 100% (relative) and you get condensation and "wet water".
...and/or dirt on PCB may cause considerable "leakage" on such high impedance signals.
Certain solder fluxes can be conductive.