If you want to peg coins to something, you need to get real-world info into the chain to adjust supply. There are two ways to do this: trust someone, or do it by vote and trust voters to do what you want.
In other words, your proposal is effectively the same as "let coin generation parameters be setr by vote" and "hope people will vote to peg prices".
I described a scheme for doing this here:
http://forum.bitcoin.org/index.php?topic=24929I plan to build such a system after meeting work deadlines, about 4 weeks.