Yeah we've tried this - I posted this idea way back to try and gather metrics for this.
Our previous implementations on this employed 3rd party analytics to track custom events / activities and then we created a set of endpoints to allow Khoros to pull in this data back in to create our custom ranked leaderboards. It worked OK, was a bit slow and kinda lagged due to the turn around on the metrics.
A lot depends on what you want to measure for the leaderboard, but off loading data to another system seems like the only option unless you can derive these metrics by calculations or simple logic on the existing data.
