What I've written so far implies that there will be a DSL attribute on project called `mac_pkg_identifier`. I chose to tack on the `mac_pkg` prefix because "identifier" is a pretty vague term when there is no context.

If we do offer a fallback when it's not set, I'd like to choose something that's not likely to conflict if, say, the upstream releases an official package. All of the packages I have installed follow this reverse domain convention. There are some reserved domain names, so we may be able to use those when offering a fallback. The full list is here: http://www.iana.org/assignments/special-use-domain-names/special-use-domain-names.xhtml but these are most promising:
* example.com
* example.org
* ANYTHING.local
* ANYTHING.test