I was lucky enough to have a mentor from the Santa Cruz New Teacher Center oversee my BTSA process and the relationship was much like the one you propose. My mentor worked for SCNTC and so she was able to devote all her time and energy to supporting new teachers and she wasn't affiliated with our admin or district office. I'm not sure how this could be implemented widely, but my experience of it was incredible and probably responsible for my survival as a new teacher!
I love this idea. I have a friend at San Francisco Unified who took a year long sabbatical and seems totally reinvigorated by it. I will check in with her on how structured her year of exploration was and if/how she was able to share what she did with her colleagues.