What occurred to us is that soon, using MasterCoin backed currencies feature, we could implement such indexes as trustless independent assets! (assuming the whole backed currencies feature works, which still remains to be seen).
We could program the creation of an asset, backed by a Trust Fund, that aims to track the index itself.
This is how it could work:
1. We would publish price feeds of the various alt currencies on top of the MasterCoin blockchain.
2. We would then create a new backed currencies, backed by Mastercoin for example, that tracks the weighted average of the top 10 currencies in our feed.
Regulatory wise, this might mean that we are no longer "creating an index" on an ongoing basis, because what we'll be doing is just publishing a price feed of the alt currencies, not managing the composition. The price of the new index coin will be automatically calculated by the Mastercoin blockchain (or rather the clients implementing it), and the fund would purchase or sell coins based on the discrepancy between the price of the new coin and its underlying assets, just like any other backed currencies.
TL;DR - if "backed currencies" can be used to implement GoldCoins, they can be used to implement IndexCoins.
I am interested in this field. Would like to talk to you all and learn more to, and explore if there is something I can contribute.