We've had success with this approach with outside recruiters. Generally the recruiters aren't skilled enough to coach a candidate on the tasks. But in the case that they are, one of the components of the onsite interview is a code review of the code the candidate wrote. This usually includes me challenging design decisions and then changing up requirements and asking how the code would change. So even if a candidate WERE to be coached by an recruiter, the candidate would have to really understand the code they wrote and why they wrote it.