This would work if the service is stateless (ie owns no data) or the state that it carries can be duplicated without ill effect. A lot of services won't be like that though.
That's a good point. You'd need to figure out what to do about persisted data the service depends on as part of the forking process. In the cases that aren't such that you described, I'd guess most of the time you'd be stuck trying to get your changes merged by the home team - something I think is much more desirable for long term code quality (in theory)