Since this is all a black art and guesswork at best and pseudo-science at worst... The most obvious and simple method is to get an average PageRank of all page

Since this is all a black art and guesswork at best and pseudo-science at worst... The most obvious and simple method is to get an average PageRank of all pages and if it's higher than average use that figure to bias the results as 'domain authority'. I think there is some sort of bias towards newness, so more recent links are worth more, and newer sites are biased to have a higher initial PageRank.