There are microcontrollers with I2C built in. That would be the easiest for you, since you already have an I2C interface.

Microcontrollers respond very well to contact closure, etc. you probably would want to have a ramp up/down set of switches for each function. The LCD could then be set to indicate what was being adjusted, and to present a progress-bar of the current value.

You probably should get yourself a primer on microcontrollers, because, though they are not tough to use, the implementation is a bit too much to post in a forum like this...

I would really love to have a look at your circuit diagram. Would that be possible?

You could send me whatever you are willing to send to wrenb@integrityol.com

Brian