Why not with a Browser plugin?
Pidgin already Supports OTR via gtalk. And on Android you can use Gitterbot.
Google should really implement OTR in a Javascript library in the Client Browser. Would that be possible?
Sure, but I like the convenience of web-based no-install gtalk. I think as far as google is concerned convenience beats security.
They "read" your mails, know what you search for and who your friends are. It would seem strange for them to go out of their way to provide truly OTR gtalk. And I'm totally fine with that, there are plenty of alternatives if I ever need encrypted chat.
