The feasibility of I2C depends, in large part, on how far apart the Arduinos are going to be from one another: the I2C bus is not designed for long runs over cables. Plus, since this is going to be part of your 256-motor project, it could turn out to be an electrically-noisy environment.
My inclination would be to put an RS-422 or RS-485 transceiver on the motor interface board you're going to be building: it's cheap, simple, designed for multi-dropping, and very noise-immune. And could well pay for itself in reduced debugging time.
If you use something like a 75176, you can easily add the ability to talk back over the same twisted pair if you want it later.