We we're wondering if there was a way to expose the fact that the underlying hypervisor is down? Something like a "Warning : no data from compute since xx:xx:xx" I did not see any documentation regarding a possible configuration option somewhere but a lot of posts with people with similar questions.
You're looking for "host_status" in the detailed server output. It gives you an indication of what the state is of the host the instance is on without revealing too much and without altering the state of the instance itself, which as you note could be wrong if the problem is merely communication.
https://docs.openstack.org/api-ref/compute/?expanded=show-server-details-det...
This is controlled by policy and only visible past microversion 2.16, so make sure both of those details are handled for whatever users you want to be able to have that level of vibility.
--Dan