But there aren't any meaningful stats you can extract out of the block chain. The fact that the block chain exposes who paid how much to whom is a bug, not a feature, and that source of data will be eliminated in the near future. The question will become: how do you control the nominal interest rate without any econometric data whatsoever?

There's a reason Freicoin's demurrage rate is flat: it's not out of laziness or ignorance. It's the best you can do! The only other real option is to track by means of a synthetic asset or prediction market, but that leaves the entire economy vulnerable to collusion and manipulation. But having a fixed demurrage rate doesn't make Freicoin broken - instead of the real nominal interest rate varying between 4-6%, it'll vary between -1% and 1%. This is an improvement. And, sadly, the best that can be done without sacrificing user privacy and/or decentralized control.