Steganography is all about hiding messages in plain sight.
Theorem 1.1: Surveillance is like finding a needle in a haystack.
Theorem 1.2: You don't even have to disguise your needle if your haystack is big enough.
Therefore I propose a secret language with a restricted vocabulary of a few hundred words, like Basic English - the difference being that the words chosen for this language are exclusively top ranking search keywords and brands, and therefore ungoogleable. Here's a secret message that will forever be lost in the maelstrom of noise that is the web:
"Facebook maps ford insurance google web iphone social? Linux amazon twitter 2.0 sony microsoft!"
Every branding and marketing campaign will assist in driving the secret further into obscurity! Keyword-sniffing filters and miners will be up against a $500bn industry which fights to drown out any subtle meaning attached to brands solely in favour of their own chosen meaning.
(Obviously, forget defining a new natural language; you'd just encode your secret document with a byte-to-brand map using, say, the top 32 global brands and base-32 encoding).