I think this is a good idea -- however, in my experience 90% of the issues with Jrs is they don't yet know how to ask questions. So much of mentoring involves finding ways to encourage them to speak up. Once that's achieved then the amount of work required to get them up to speed feels much less steep. Further, teaching Jrs how to communicate is largely a company culture issue where openness is a given. Lacking that, with even the best mentor the Jr will struggle.