We had exactly the same problem, so we built a bot for our Engineering room in HipChat that pesters the entire room about how many pull requests are currently o

We had exactly the same problem, so we built a bot for our Engineering room in HipChat that pesters the entire room about how many pull requests are currently open, what they are, and links to them, once every 2 hours (during business hours). It's worked really well. We've also extended that same bot to do other things like remind you about support requests, etc.