Your going to have to do that bit of programming. Your going to have to create a bunch of I2C commands on your slave. Then your master can send these commands over I2C. Your slave in the meanwhile will keep polling and when it receives a command it will carry out what you want it to. In this case it would run commands off the liquidcrystal library.
Here is some more info on creating a simple i2c slave/master: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1208761098