What if a reviewer is on vacation or sick leave? Who can step in for them?
It is not currently possible for the HR person (nor the manager) to step in for a person who is away. It is possible for HR and managers to add themselves to the reviewer list of the reviewee (clicking the "manage reviewers" link on the 360 overview) and then act as a regular reviewers.
In the feedback, an HR person or manager could then write something like "the following is on behalf of Joe", for instance.