Gremlin, out of curiosity... Are you using pwms to dim backlighting of panels? If so, I would de xpect I2C to be more prone to picking noise on the data lines (as it is a single ended bus). RS-485, on the other hand, is a robust bus conceived for industrial environment: being differential, has a bigger immunity to common mode noise, and was specified for longer distances than I2C.
From my personal experience, you may runinto some problems with I2C when using long wires, so pull-up resistors might need to be adjusted. However, noise might be more difficult to address...
That being said, I fully understand your reasons to go I2C, and also think it is the easiest way so, if it's working, don't touch it!!! ;) I only wanted to lnow if you were using it with backlighting-pwm. As I said, just curious :)