I recognize that it would be impossible to do this precisely, but you could at least come up with an estimated number for how often a particular item goes unsold on the auction house.
Couldn't you just keep track of the currently available number and watch and see if that number ever went down by a larger amount than the recently sold ones showed?
For example, if you watch it and there are 5 available on one scan. Then on the next scan there are 2 available but only 2 additional ones on the recent sales, you could conclude that 1 out of 3 didn't sell.
The imprecision would come in if an item was returned unsold and a new one was put up before you scanned the histories again, but I dont' think those few occasions would be that big of a deal.
So yeah, to sum up, I think it'd be cool if I could look up an item, see a price, and then see that around 80% of the ones put on auction house don't sell, and use that info to know that I should lower the price.