I have not yet played with the ESP modules. My main concern is that anything using wifi burns power, and with my current system, sending a few integers every minute, I've gone over a year on two AAs (I replaced them recently when trying to debug some issue or other). I wanted to get an ESP8266 to build a single-use button recently, but I ended up just buying an Amazon Dash instead and hacking that.
Jules, I did a "literature search" (to give some Googling a dignified term), and found this:
https://www.gazelle.com/thehorn/wp-cont ... covery.pdf
See page 8. Rice works, kind of, but instant rice (or oatmeal or couscous) is better. That's just absorption over a 24 hour period, though, and maybe the equilibrium RH performance would be better or worse. I'll have to find somewhere to put my real desiccant in the meantime, though, and all my rice arrives in paper sacks, so it's not as pristine as Ed's.
Great print, by the way. Did you use vase mode?