I just signed up for the trial of CircleCI, not least because I’d like to use it for an Open Source OS X project of mine. Most of my projects are hosted in Bitbucket, rather than Github, primarily because I prefer Mercurial to Git. Unfortunately, on the “Add Project” page, when I click on my Bitbucket team, I see “No matching repos for organization”.
mercurial isn’t supported
That would certainly explain it, but you can’t claim to support Bitbucket without supporting Mercurial. Bitbucket was originally Mercurial only!
You could create a Git mirror of your Mercurial project.