Frankly, I never tried (I rsync to another server for obvious backup reasons) but I suppose that if you configure the right rsync listener locally, you should be able to achieve it.
The reason why I hacked Zentyal script is that Zentyal syntax is not convenient (from my standpoint) when it comes to RSYNC.
This being said, behind this simple initial question about RSYNC backup to local drive, there is another wider debate about Zentyal backup usage and purpose that will, if launched, lead us to another debate about availability, redundancy...
I wonder how main Zentyal users handle it