We have been doing house-swaps for 19 years ( this year will be our 40th).
We are members of Homelink which is the biggest home-swap organisation and will provide you with the best chances. I assume you have tried them ?
HomeLink Australia - International Home Exchange
Home Exchange with Intervac - Our people, services and global reach
Also you can try Intervac ( link above), the second biggest :
Homelink will cost you about $200 for a year and is really worth it. They have a system of references you can ask for and we have never had a bad exchange.
I am amazed people don't do this as a matter of course for their holidays as well as sabbaticals.
You can also swap you car as well. Many members are looking for long-term exchanges too such as six months and over. For your yearly membership fee you can do as many swaps as you wish anywhere around the world too, including the country you live in for shorter ones.
Otherwise why don't you try the Harvard University Gazette, I know people here in Oxford use it for the same advertising purposes ( ie : sabbaticals).
Good luck and enjoy Boston, it's a wonderful city and New England is Fabulous !