This might be a possible fit:
http://www.magic-software.com/Applications.html
(see "Boolean operations on 2D polygons. Based on BSP trees" near the
bottom of the page).
The license agreement might be acceptable. I'm not a lawyer, so I can't
be sure how free it really is to use from reading this:
http://www.magic-software.com/License/free.pdf
It would probably be good to contact the author.
http://www.magic-software.com/CompanyInfo.html