Exchanges could broadcast a message to the network, signed with their private key to ensure authenticity.
Incredible idea! That way, the exchange rates could be published through the network itself, instead of having to rely on centralized auxiliary sources.
Maybe you could follow this idea:
http://forum.bitcoin.org/index.php?topic=8773.0In short: allow anyone to publish an time-dependent exchange rate to another currency through https, and map changes in the exchange rate to virtual "interest" transactions in the client view.
Yes, I think this would be similar to my idea; at the time of a transaction coming in, attach the current exchange rates to it as client metadata. There'd be some problems with this approach, though, for example what if you don't run the client for a while. You'd need historical exchange data to reconstruct the exchange rate at the time of the transaction... unless you really embed the exchange rates in the block chain, but that might be overkill and a waste of space.