Thanks for the suggestion, Thomas. I agree that this would be a great feature for the HipChat service. I've added a ticket in our internal tracker and will be leaving this issue open for additional community feedback.
I second Thomas. We are evaluating HipChat as a communication platform for developers, we are already a customer of BitBucket, would be nice to have fine grained control over what notifications are sent over what channels.
According to my testing, none of the services (e.g. POST) receive notification about a newly opened pull request, comments posted, or merges made. This seems to be a general feature request towards BitBucket, instead of being only HipChat specific.
FWIW, with no out-of-box way to do this with Bitbucket/Hipchat integration (like there is with commits to a repository), we are using Zapier.com to accomplish this. Zapier looks at activity in bitbucket and posts to a HipChat chatroom on 5-minute intervals. We have it setup to post pull requests and approved/completed pull requests.
FWIW - I wrote a nice little webhook proxy for Bitbucket -> Slack that could probably be adopted easily for HipChat, where the existing integration falls short. Relies on AWS Lambda for spool-as-needed proxying.