Well I simplified it by just using the sketches in reply #8. Simply sending the messages out straight from the arduino. Mikal has confirmed that there doesn't appear to be anything wrong with the logic - "Logically this seems ok. Are all devices grounded? All operating at the correct voltage? There isn't any RS-232 anywhere, is there?"
However I suppose there is RS232 by way of the level shifter..
I'm not sure that there is much left to try, I'm sure the LCD device itself is fine because it works through the dedicated UART port through the level shifter just fine, and through the PL2303.
The only other thing to try is a .22uf cap in place of C1 of the charge pump required for the Max 3323 which is specified in their datasheet but this is not what is shown in the arduino rs232 tutorial - http://www.arduino.cc/en/Tutorial/ArduinoSoftwareRS232
I'm not convinced this would actually achieve any different results though as the existing circuit (wired the arduino rs232 tutorial way) works through the UART pins.