The only other attempt I've seen to algorithmically calculate the difficulty of this sort of puzzle is http://www.latinsquares.com/Explain.html#difficulty - the Latin Squares puzzles generated there are like Su Doku (never been quite sure how that pluralises) except that the boxes can be arbitrary n-ominos.

Thank you, by the way, for writing all this software and distributing it freely - the Su Doku solver is extremely powerful and very pleasant. Can I point you at the World Puzzle Championship (http://www.worldpuzzle.org/) which has a very wide variety of types of this sort of culture-free logic puzzle? I think you'd enjoy the puzzles therein.

My Mao preference was for lots of fairly easy interlocking rules, but I have strong preferences in Mao (and Nomic) that are at the fluffier end of the scale, sometimes attracting disdain from the hardcore intellects. :-)