It would be convenient to drive the displays from the Vin pin on the mega, but sounds like there would be some challenges. So I decided first to do a simple test using an Uno and one of the large displays. The idea is to use the Uno to control the display, but use a separate power supply to power the display. I hooked the display up to the Uno using the appropriate I2C pins. The Uno is powered by USB. For power to the display, I connected directly to a 4.6V (measured)/0.775A power supply. I used a simple sketch which just counts cycles. The display lights up (displays 1234), but it seems the Uno never takes control of it, and the display never changes from 1234. Letting the Uno power and control the display, the sketch runs fine. I tried a few other things, but the bottom line was the same....the arduino (uno or mega) I2C never seems to take control of the display when the display is powered by an external source....it just sits there at 1234. I have a feeling I am missing something obvious, but I'm currently stumped...it seems like this should work.