Statistics for posts/comments by user across MS Install

What I’m looking for is, minimally, a way to track the number of posts and comments that a user makes across a multisite installation.

There is a plugin out there (http://wordpress.org/extend/plugins/participants-widget/) that tracks and displays the number of posts and comments made by a particular user in a particular blog. I’m asking about doing that for a blog network.

I asked the question in the hope that there is something out there — does not have to a plugin — that does this already.

Any thoughts on how to approach this?