Never spell the same word twice?

A long time ago I was reading about Renaissance-era ciphers and I remembered this quote:

David Kahn, author of The Codebreakers, quotes Giovanni Battista Porta who published, in 1563, a famous cryptographic book, De Furtivis Literarum Notis:

"He urged the use of synonyms in plaintexts, noting that 'It will also make for difficulty in the interpretation if we avoid the repetition of the same word.' Like the Argentis, he suggested deliberate misspellings of plaintext words: 'For it is better for a scribe to be thought ignorant than to pay the penalty for the detection of plans,' he wrote."

The last sentence seems to be a clear reference to the concept of plausible deniability. But I am more interested in the technique of deliberately misspelling words. Is this a technique that could potentially be effective against modern mass-surveillance systems?

For example, in an NSA-style mass-surveillance system that flags certain designated keywords, could deliberate misspelling of words potentially be used to defeat that system? Or is it more likely that such a system would already have methods in place to detect these misspellings? (For example, something like Google's "Did you mean to type x instead?" feature.)