At a lot of jobs I've dealt with using contractors, there was an internal concern that people external "didn't care" enough. How do you plan on addressing this concern?
It's a system that will depend a lot on trust, and it may be difficult to build that with companies whos DNA does not lend them to trust outsiders AT ALL let alone let them come in and muck with production level code (letting juniors do it!)
I'm playing devils advocate, like I said I think your idea is cool and worth exploring. Not enough companies invest in juniors to grow them into their full potential, and I commend you for this effort. Good luck.