As usual we are on the same wave. Some weeks ago I started digging into the same area. So far I have not found anything ready and therefore started some research and it seems that we have some cornerstones already supported by the platform. I believe it would be possible to build up a service like that based on the vTPM (
https://docs.openstack.org/nova/latest/admin/emulated-tpm.html), SPIRE and most likely an additional custom component for the integration. Actually I was thinking about the problem from the next step point of view - how to provide the Keystone API key into the VM and thus came to the very same problem statement.
Maybe somebody has already built a solution for the attestation. Otherwise it makes sense to start thinking deeper on that.