You can use SQL Backup Pro to back up AlwaysOn Availability Groups, including taking a backup from a ReadOnly secondary replica.

You can use any of the following options:

• use the option CHECK_PREFERRED_AG_REPLICA to ignore backing up on non-preferred replicas. No backup will be taken on non-preferred replicas, however the backup job will succeed.
• use the option SECONDARY_REPLICA_COPY_ONLY to apply the COPY_ONLY flag for full backups on a secondary replica.
• use the options SECONDARY_REPLICA_COPY_ONLY and CHECK_PREFERRED_AG_REPLICA to schedule the same backup job to run on every availability replica. This means that, when using a shared location, it doesn't matter which replica is the primary replica. The database can then be restored without knowing which server was the primary replica.

For additional file name options see File location tags

For more information about the types of backup allowed on Availability Group primary and secondary replicas, see AlwaysOn Availability Groups (SQL Server) and Configure Backup on Availability Replicas (SQL Server) on MSDN.