well, it really depends on what you mean by 'mentoring', if it means answering questions, SF already has that facility. I, for example, ask questions relating to my GCSE course. If it means teaching, then, it would be really difficult for the two, or more members to meet at the same time over here, because of the fact that SF is very wide-spread among people in different countries.
This is just my opinion on this, but, its a great idea. The thing that may stop this from happening maybe the difficulty in putting it all together