Assuming contracts can identify the coinbase address of past blocks, then that would be relatively straightforward. But this may not be up to the miners alone.

Assuming contracts can identify the coinbase address of past blocks, then that would be relatively straightforward. But this may not be up to the miners alone. Perhaps even more important is what for the major exchanges choose to run, since they're needed to convert Ether back into BTC/fiat currency. If a major exchange makes a public announcement committing to either the soft or hardfork, then that will sway a great many miners.