I remember finding an article on HN about a developer for FreeBSD going to work on Mac OS[1], before returning to work in open source. Isn't the Mac OS user space an example of what you describe?
I don't think I'd say it is, no, because Hubbard wasn't hired to work on a closed-source fork of FreeBSD at Apple but rather as an engineering manager. AFAIK, Apple did not make any closed forks of major BSD-related work.