3474776 - Process Documentation and published & unpublished Revisions of a model

Can a Process Documentation Template be used on a published model, when there is a later Revision that is unpublished?

No, Process Documentation can only be used on the latest Revision of a model.
This includes the unpublished Revision, when an earlier Revision has been published.

In this scenario, the only way Process Documentation can be used on the published Revision is to restore the published Revision in the Feed.
However, there will also mean the restored model will be created as a new Revision.

For example, if Revision 1 is published, and Revision 2 is unpublished, then restoring Revision 1 will create Revision 3.

KBA , BPI-SIG-PM-DOC , Process Documentation for SAP Signavio Process Manager , How To