If you are referring to the current version: OpenSprinkler 3.0, the expander uses I2C to communicate with the main controller, so the question is really how far I2C can go. I did some Googling:
http://www.esacademy.com/en/library/technical-articles-and-documents/miscellaneous/i2c-bus/frequently-asked-questions/i2c-faq.html
and it seems going 100 feet is definitely possible, by reducing I2C clock speed.
At the moment, the firmware uses typical fast-mode I2C speed, which is 400kHz:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/blob/master/SSD1306Display.h#L22
it can be lowered to any clock speed you want (though below a certain number you may also need to modify Arduino’s I2C library because it may be imposing a lower bound on how low the clock goes).