I've had this in place for a week or two now, it's working well. I updated to the new (sleep mode) release yesterday and so far, so good (I have a breadboard version at home that has had a bit more testing).

I'm publishing this now and will see how it works over the rest of the season.

I may return to it since the PCB is not brilliantly laid out, and I'd like to reduce power even further by powering the DHT11 off of one of the ATtiny's unused digital pins.