It works, but only under the assumption that user liveness is constant or increasing over time. A liveness metric should differentiate strongly between a user who has commented 30 times in the past 4 days, and someone who commented 25 times in the first 4 days of news.yc. and 5 times since. Perhaps I'll hack something together.
lynx -source http://news.ycombinator.com/user?id=$USER_IN_QUESTION | grep '<span class="comhead">' | wc -l