I've created a patch that adds support for the before/after_add/remove callbacks to belongs_to. I found an old patch that was only partially complete here: http://dev.rubyonrails.org/ticket/6934
My updated patch along with tests can be found here: http://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/1564-add-support-for-before-and-after-addremove-callbacks-to-belongs_to-associations
Thanks,
- sob