While the restrictions have gotten tighter in Philadelphia, I know of a wonderful OB there who regularly serves as back-up to home birth midwives. I don't know the arrangement she has with her hospital. Is there someone similar in Boston? Feel free to send me a private message, if you wish.
I'm in Boston as well, planning on a HB for our first this fall, and also in training to be a CPM. I haven't yet heard of any Boston OB's backing HB mommas, but according to my midwives, Brigham & Women's is generally pretty good about HB transfers. Sorry this isn't more helpful!
Best of luck to you!