Right, that's half the problem. If you could figure out a reliable way to capture the value of your coins you could hook up the money supply (in Bitcoin's case the block reward) to it, but that seems to be impossible (?) without some trusted authority somewhere.

The other half of the problem is that you probably need a way to actually take coins out of circulation if the value of your coins drops. This is particularly true if you've just taken away the upside to holding your coins by making a protocol that prints money to stop it appreciating.

And once we've done those two things, we seem to have reinvented the Central Bank. Which is why the best we could do as far as I can see is to still have multiple Central Banks, and the ability for the community to make a decentralized consensus decision to blacklist any that might misbehave.