I don't see how this can be practical when it generally takes at least a week of ramp-up before a new hire has gained enough familiarity with the tools and codebase to do anything that could be called "real work".
While that 1-week (or longer) ramp-up is generally true for companies, it's clearly not true for most (successful) open source projects. The things that a good open source project has (good documentation, onboarding, tests, etc.) are also the things that a good work sample will have.