Can you describe in more detail which mechanism you would like to use and what exactly you want to achieve (who shouldn't be able to access the data)? How is your environment set up with regards to transport-level encryption?
Uhhh, i'm not 100% sure, because i never heard of such a request. But as far as i know DSM does not react well on modifications of the underlying SQL Database. Especially when the access is restricted. Wouldn't be a surprise if your system doesn't continue to work after that.
What kind of Data do you store in in the schema extension to have such a request?