I don’t think there is a direct way to automatically detect the LCD — unless if you use an I2C type LCD (in which case checking the existence of the I2C address will allow it to automatically detect LCD). Most of the code for LCD is already in the firmware code, it’s just that they are only enabled for the microcontroller-based OpenSprinkler. The OSPi branch disables all LCD functions. You can do some copy pasting to basically replicate the LCD functions to the OSPi branch.
To reduce the number of wiring, I do highly recommend you to use the I2C type of 16×2 LCD (just search ‘I2C 1602 LCD’). It only has 4 wires so is a lot easier to hook up to RPi.