Is it possible with rsync to not create directories on destination?

Imagine I have that source :

a/
a/x.txt
b/
b/y.txt

And that I have this destination :

a/
a/z.txt

The wanted result of rsync source destination :

a/
a/x.txt
a/z.txt

Of course my real situation involves thousand files/folders structure and I don't want solutions involving explicit list of synced folders, which I can do. I'm looking for a clean way just to prevent any folder creation on destination. By exclude or filtering... That could even be something outside rsync, like a hack with permissions if rsync can't do this...

For information, this is really easy to get this kind of situations, in my case I have:

• A server with 2 disks, let's say A & B. And a local drive C.
• I usually use rsync to sync (and merge) remote A & B into local C.
• Then sometimes I just want to sync back some C files into A and B. (Just new Files... not non-existing folders on destination)