The equivalent of SIGSEGV/SIGBUS on Mach is handled in basically the way he describes. On an access violation, it suspends the thread and delivers a message to the registered port. The thing listening on the port (in a different thread or even different process) receives the message, does whatever it needs to do, and then sends a response, after which the original thread is woken back up. From the perspective of the violating thread it was handled synchronously, but the actual implementation was an async message queue.
It is not much better though. If it is another thread it has to work under the same async signal safety rules of a signal handler (the blocked thread might be holding an arbitrary mutex). If it is another process, there isn't a lot it can do.
