Cool idea. I don't see any reason not to support CORS; all the OpenStack requests require a custom Auth http header anyway, so I don't think there's any danger in turning on CORS there. I didn't see anyone in the Launchpad bug report that was against it.
The only call that I think _might_ be dangerous to expose is the login API, but that should be rate-limited and / or have lockout anyway.
I've opened a bug to support CORS in FathomCloud. It's not a lot of work (I already have the filter in the repo, it's just not configured in). I just want to think it through to make sure it's safe: https://github.com/fathomdb/cloud/issues/50