The funny thing is, I'm pretty sure the HipChat client is (or at least was) mostly just a shim around a web browser with some notification stuff bolted on. This doesn't need to be a huge undertaking.
Maybe it's changed since I last looked, but they didn't have a client at all. You could download Chrome and then install an "app" through the Chrome store. Well, some of my users prefer Firefox... I don't know, it just wasn't a great onboarding experience.
In retrospect, though, I might've gone too far the other way -- hipchat is pretty bare bones.