Close issues with a commit from a separate repository

I have a Project on BitBucket, and the project has a private source code repository in addition to a public "issues" repository. I don't want users to be able to see source code, but I do want them to be able to file their own issues about the project, hence the separate repos.

It would be nice to be able to close an issue by committing something like Fixes teamname/issuerepo#issue to the source repo. Currently, the commit message just contains a link to that repo, but the issue itself is not linked.

Another solution would be to have a way to designate an "issues repo" for any particular source repo, so committing Fixes BCLOUD-10 would close Issue 10 on the "issues" repo (this, of course, would require that the source repo not have issue tracking enabled on itself).

This would also allow multiple source repos to share the same issue tracker.

EDIT: I see now that "Public Issue Tracker" is an option for a private repository (this wasn't available when I first created the repo a while ago), but this proposal could still allow cross-repo issue closing (which GitHub already supports) as well as a shared issue tracker for multiple source repos.