That is an interesting approach. But the simplest encryption (e.g. a simple XOR) will go around this problem very easily.
A prescriptive signature-based black list, as you point out, is easily fooled with simple obscurity.
An additional twist that seems daunting (but interesting) is to mark sensitive data at EVERY step in it's processing, with support from the OS and hardware, and never let out tainted data out without explicit permission. See Perl's tainted variables for the gist of the inspiration.
So if a = "User's name", which is protected data, and you do b = a, then b is tainted, too, and write(socket_fd, *b) would pop-up an alert.
All old hat, I bet, to security researchers. I'm just thinking out aloud.
It sounds good, and because compliance is about by making good-sounding things mandatory (weekly password rotation, yay! /s) it got mandated in a lot of places.
And it did catch mistakes, like accountants sending the wrong files or to external addresses. Which I guess is justification for it right there.
But it's billed as a stronger (ie hacker) protection, for which it's useless, so I never liked it.
I think the world would be safer with an email plugin that helped you by suggesting that you should not send a document to a given address, based on rules and observations. It'd only be a suggestion so nobody would expect miracles, but it'd stop all the unintentional mistakes our system stopped, for a fraction of the price.