Hi OzMan,
When dealing with text substitution it is a inverse relationship between human readability and the amount of substitution. Even with the most advanced lexical and symbolic text generation systems the output tends to be unnatural prose. There are a couple of fields of artificial intelligence research that may interest you regarding this matter: Natural Language Processing and Natural Language Generation. I've investigated into them both and with the former I've found that the most advanced systems right now can do nothing more than (what it seems) template parsing.
So in short, parsing text on the conceptual level, processing it using a symbolic or lexical system, and then constructing the text with a text generating system is something I'd personally love to have, but I think we are probably 10-20 years behind on that one.
The rewriting engine can be used for very specific words and phrases, and is very effective in changing the *style* of the chosen text while preserving the context.
~the Ark