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.
Perhaps I was a bit hasty in my discussion of Freicoin previously. If you could get an interest rate of -1% to 1% then without demurrage that would be an unambiguous improvement, since it represents a lessening of the inflation tax on holding money. However, doing it via demurrage is just replacing the inflation tax on holding money with an explicit demurrage tax. The fact it's explicit makes it marginally preferable, but the economic distortion is still there. People have to hold money to perform transactions, but they're penalised in doing so by the presence of inflation/demurrage.
I started working on a paragraph describing a potential mechanism, but I see there have been several subsequent posts, so I will defer it till my next post.