https://community.synology.com/enu/forum/17/post/72559
"I'm thinking that we should create a user on our NAS for each other, with a data quota, and that user only have access to a given folder. I suppose I will also encrypt the files on my NAS... and assuming that he does not have my encryption key, I do not think he will be able to read my files, although he is admin on his system. But I'm not entirely certain how to implement this, so any advice would be appreciated."
so yes it can be done, but this does not look like its automated like we want