Hi.
Just published current status of my work on ATTiny85 node with NRF24L01 and BME280 (4 pin I2C version) and want to share it here.
https://github.com/rmrfus/tinybme
TLDR - It works! 
It is based on fabyte's work and couple of other resources.
- NRF24L01 works from 3 pins - MOSI/MISO/SCK, keeping CE constantly high and multiplexing CE over SCK.
- BME280 is connected via pins 3 and 4 on ATTiny85 and communicates over SoftWire
- It works just fine from 18650 battery. Prolly should work from CR2032 too. Didn't test current consumption (yet).
- Right now code is just 92 bytes under 8K limit - no space for conversions of units - I'm doing it at HASS.
- There is neither free pins nor flash to report the battery status. Prolly some kind alarm on lost heartbeats is much as could be done in this situation.
In general I'm open for suggestions and improvement ideas.
