Oh... kay....  I'm game, but still struggling to see how to do it.

First explain carefully what a "repo market" as opposed to, say, a bond exchange or stock exchange or futures market is, or when one or more of the latter counts as a repo market.  Second, exactly how an interest rate relates to such a market.  The only one of those market types where interest rates are directly measurable seems to me to be some kind of futures market. 

If it's a service that we can figure out how to completely decentralize and build into the blockchain itself, then we're cooking with gas. If it's something that would be an external resource, or that would have to be centralized at a website somewhere, then it gets us nothing. 

The basic requirements here for the automatic reaction that we're looking for is that it has to arise in a decentralized p2p service, where the information that's being reacted to is visible to and checkable by literally everybody who can see the blockchain up to the point where the information matters.  Essentially the problem is that if it's an external source of information - if it reflects information not immediately visible to everyone looking at the blockchain - then people will not be able to check the blockchain to see that the response to it was correct at every point.   Being part of the blockchain, in turn, means it has to be a fully decentralized P2P application. 

Now, if it "still doesn't seem too hard" to you, then obviously you have a lot of experience designing fully decentralized P2P applications.