You asked ‘Is it possible to use the ESP8266 (or similar) to become a remote switch to control a valve/relay?’ — assuming I understand it correctly, I think the answer is yes. First, OpenSprinkler 3.0 uses the same OpenSprinkler firmware API:
https://openthings.freshdesk.com/solution/articles/5000716363-api-document-firmware-2-1-7-current-
you can use a script, or a program, or a command line tool to send command to the controller to trigger a zone or a program. I assume that’s what you mean by ‘remote control’.