In this thread in case you haven’t noticed.
http://www.bulletphysics.com/Bullet/phpBB3/viewtopic.php?f=11&t=1350
I take interest in this function (though the docs. in the links in that link say to not use it for now). It sounds like soon we’ll be able to have a single joint breaking with enough force function
setContactBreakingTreshold(float breakingTreshold)