Thanks for the #feature-request @Danmiser! 
I’ll copy my answer from the original thread (Bitbucket webhook - (base) Mercurial support), so serve as a summary for others about the state of Mercurial support:
For now what works: webhooks from Bitbucket will trigger a build (Bitbucket webhook - (base) Mercurial support), but you’ll have to roll your own “clone” step, the Git Clone step won’t work with mercurial repos.
Setup: register any git repo (e.g. https://github.com/bitrise-samples/sample-apps-android-sdk22) using the Other / Manual option. Once the setup is done you can change the “REPOSITORY URL” on the Settings tab of the app to a mercurial one, and register a webhook manually on Bitbucket, using the webhook URL from the Code tab of the app (http://devcenter.bitrise.io/webhooks/).
If it’s a private repo then you’ll also have to use the SSH clone URL and register the SSH key (you can find on the app’s Settings tab on bitrise.io) into your Bitbucket repo (http://devcenter.bitrise.io/adding-a-new-app/setting-up-ssh-keys/).
After this new commits should trigger a build, but you’ll have to replace the Git Clone step in the config with your own Mercurial clone Script step, utilizing the Environment Variables shared by Bitrise.io.