The situation is an INVITE sent and forked to multiple branches to send to multiple devices. After forwarding the AP goes down before either the 180 or final response from any branch. In the case with multiple branches going stateless wouldn't work because the caller would potentially receive multiple final responses? And sending cancels to the other branches after a final response wouldn't work since we wouldn't know about them.

I was envisioning something like what you said, serializing the transaction info and saving it (and updating it on branch replies) and then having other hosts able to read it if they end up getting a message for which they werent in the original route.