I just finished a preliminary mockup of the circuit in Fritzing. A couple of things are still missing. I'm likely going to add a couple of switches (mosfets?) to turn off the servo and ESP8266 when not in use. Of course, the moisture sensor is just some random I2C part I re-used. The MSP430G2 will also go into a DIP socket. No need for the Launchpad.
The ESP8266 is not represented correctly. I stole the idea and the wiring from http://robinsonia.com/wp/?p=378. The retro browser project seems to have an ESP8266 schematic. Maybe they're going to release that one day.
I'll also add some connectors so I can disconnect the moisture sensor and the battery pack. Also some LEDs and test pins so I can see/check if the components are powered.