Your approach is sound. It just sounds like you need some assistance how to write the backup script so that it works with DFM / Protection Manager.
Yes, finding the documentation on backup scripts for DFM / Protection Manager is not easy to find. However, there are a number of existing discussions on the communities site that answer your question. In the first one I'm posting, Adai has posted just about everything you need to know about backup scripts. In the second posting, kutner has posted an excellent backup script (in Perl) you can use as an example to get started.
DFM / Protection Manager only supports native executables or batch files (.bat, .cmd) on Windows platforms. If you want to specify a backup script that is not in a native Windows language (i.e. ActivePerl), you must enclose it inside a batch file.
On Windows platforms, you do not need to specify the "run as" field. It will always run using the account that is used to start the DFM services at boot time.
I highly recommend defining your PATH variable within your batch file. This helps your backup script locate all of the commands it needs to run properly.
Log the execution of your script to a text file for easier troubleshooting.
IMPORTANT: The backup script must exit with a non-error return code. Otherwise, DFM / Protection Manager assumes something failed and it aborts the job.