Can mirroring be used to upgrade SSRS?
Assuming native mode here...

The main thing you are migrating when doing the upgrade to SQL Server 2014 is the reports, subscriptions, and then your data connections. I can't think of a reason why you would need to put this database in replication just for an upgrade. Also, while it probably could be done (and work) it is not a supported method of upgrade from Microsoft to put the SSRS databases in mirroring configuration. So proceed at your own risk.

Unless you are adding/modifying reports or subscriptions often I would not see why you could not just do a quick migration according to documentation. Perform your testing of the new version with your reports, and then at cut-over time just do a backup/restore one last time.