Heh, that's actually a neat idea. The profile of people which regularly send/receive encrypted email is basically geeks right now. Key off of the "-----BEGIN PGP ENCRYPTED MESSAGE-----" bit.
Yeah. Honestly, I think that if you know that someone encrypts their messages, you probably know a lot more about them then most other data points reveal.
It would also get Google a lot of street cred for being a privacy centric business.
My only problem is, I just don't see how you can scale the Chromebook software stack to a privacy centric business. I can't imagine trying to secure PGP keys on a Chromebook. But I haven't used one since the early beta models, so others may have more informed opinions,.