I think that is an excellent idea. I dare say some children will take to the Montessori methods better than others, and having the option to move children back to main stream and visa versa without having to change schools seems sensible. I'm in favour of streaming in schools and using a stream to provide a different technique sounds fair enough.

But then again I think Montessori methods are a side issue here.
From the parents point of view these free schools are about controlling intake to keep the riff-raff out.
From the government point of view it is about breaking the teachers' expensive employment conditions.
Sorry to be blunt, but that's my take on it.