How to fetch exchange rate in a contract?

[mmalmi]

I've been sketching a margin trading addition to EtherDelta. This would allow users to lend their tokens for interest to traders who want to short sell.

Margin accounts are backed by collateral and can be liquidated by the creditor if the market value of collateral falls below a certain threshold. Do you have any ideas for fetching the exchange rate from, say, last 12 hours in a contract? Afaik, contracts cannot access the event log. Current order book depth would also help, but EtherDelta stores it mostly off-chain.

Incomplete and untested draft:
mmalmi@0eb1c33#diff-e438becd3bae07de17d35137485d8c3b