Found a workaround that works for me, at least. I was using it for my bot, so now I have a second IRC connection that logs in as me, and watches for host notification messages. It doesn’t help for tracking how long someone has hosted me, but at least allows for notifications when someone starts.
As much as this unsupported API is used, though, there should really be a supported way to get the same information, whether it’s a REST endpoint or a PubSub topic.