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.

If the work is in any way usable to the interviewing company, definitely. We went that route for one of our work samples where abstracting out a context-mirroring scenario was going to be very difficult. We then paid the candidates for the time spent doing the 4-hour take-home work sample.

One anti-pattern here involves turning all of your work samples into week-long consulting engagements. Those are a great way to evaluate candidates, but it means you can only hire folks who are already doing consulting (or are otherwise unemployed). For some companies, that works.