@polskifacet thanks for the feedback, I’m glad to here. IMHO it’s easier to hook up a $2-$3 I2C RTC to the I2C bus – to the same pins as the LCD. Same stands for the IO Expander.
About the ESP8266: yes, it’s possible, but needs some – pretty minor – modification to the code, in OpenSprinkler.cpp, so that the HW revision is not based on an IO Expander on the I2C bus. Unfortunatelly I cannot do it now, since I’ve “messed” up the codebase a bit for another thing, but I can do it later to support that. As a B) option, you can buy a cheap IO expander with PCF8574.