I second this. I am an xoogler and we had a file inside of folders that dictated who you needed to get approval by. When someone edited code the review tool would crawl the tree to find the nearest owners file. We couldn't merge unless someone from that file approved the change request. Worked great!

Now that we have this tool I wonder if there is an API to add a reviewer. It would be fairly trivial to add this if there was an API.