Commit Policy Plugin verifies the Mercurial commit messages against a JIRA JQL query, which adds a lot of flexibity! Not only it checks if there is a JIRA issue key, but also check its "properties" (type, status, project, etc.).
It works with Mercurial (and Git and Subversion).
See the full documentation and the Mercurial and TortoiseHg specific page.
If Mercurial support hooks (which I assume yes), you do not really need a add-on to handle this.
Write a small perl/python script which can do a regex on the commit message, and for the matches do a REST call to the JIRA instance with the issue key (use a fixed username/password).
If the call is successful (HTTP 200), then allow the commit. You may also make it more intelligent by parsing the JSON object and seeing if the issue is still in open state :).
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