Could you go the other way and have the PC mount a volume on the Mac and copy its backup files when they change?
If that's to be automated in anyway, that sounds like "modifying the PC" to me, which as I said, "isn't an option".
I'm not familiar with Open Directory
This helps clarify quite a lot. I would suspect that you would therefore not be familiar with OS X Server either, which IMO, reveals a lot about what Apple considers to be the OS X way.
Why is that? Could it be that /Volumes is a special place perhaps?
Uh, yes. Exactly! It's a special place for, hmmm, maybe putting VOLUMES?!?
The longer I think about your assertion that /Volumes is reserved for the Finder, the more ridiculous I think it is. When Time Machine is backing up to a Time Capsule when no user is logged in, guess where it puts that mount point? With no user logged in, the use of the hdiutil command puts mounted disk image volumes, in... wait for it.... /VOLUMES!
/Volumes is the de facto standard OS X destination for the mounting of volumes, virtual, physical or network, for ANY application, and until you can present some documentation or evidence to the contrary, I feel your continuous assertion otherwise discredits any advice you're giving.
You can always do "rmdir" without a "-r" option.
Or maybe I could use the command made for unmounting volumes to unmount my volume?!?!
An even wiser choice is checking result codes and stopping your script on failure.
Don't worry, I'll do that too.