My idea: miners add the current exchange rate to every block.  Anyone cheating will simply create orphans, so you don't have to enforce a specific quote source; each new miner just has to agree that your quote was fair +/- a little bit (just like they agree now that the transactions you signed were accurate).  From there you just apply a smoothing algorithm to it (perhaps a mere SMA, but I have more sophisticated methods in mind) and adjust inflation / demurrage to compensate.  I can think of similar methods to handle other aspects of monetary policy in a decentralized way, but I don't want to get too far off the subject here.

If this is debunked, can you link me to where I can read more?