I think the problem with rsync might be that there is no interface to run it natively on a ROCK machine and to supply one would seem to go against Roon’s philosophy about having ROCK and Nucleus machines be appliance-like
I suppose one answer would be to SSH into ROCK to accomplish that, but at this time ROCK doesn’t accept SSH calls.
So, dunno.
Maybe ROCK software, and for that matter Nucleus software, could be updated to have the capability to automagically backup music files that are referenced in Settings==>Storage.
However, I wouldn’t wait on that solution. 
Without SSH capabilites, maybe an extension project for some curious/ambitious user?