That's a really good idea, and doesn't need any change to SGP or PHD2 at all. All that's needed is to implement a safety driver that connects to and monitors PHD2. If PHD2 reports star lost the the safety state become unsafe. SGP monitors this in the usual way.
There are a few things that need to be resolved, for instance PHD2 may report star lost because of a slew, but as using PHD2 is such a sensitive indicator it may be possible to be fairly relaxed about reporting unsafe.
Chris R