It would be quite an art to generate a legible post that contains both ASCII art and a consistent amount of entropy without containing too much entropy or being

It would be quite an art to generate a legible post that contains both ASCII art and a consistent amount of entropy without containing too much entropy or being too compressible. And since posting it adds it to the compression dictionary, it's likely to only going to work some time before it gets compressed better. You'd be fighting yourself more than the captcha. Plus an attacker would now have to, in order to figure out if a post will work or not, attempt to compress the post over X number of compression algorithms (plus of course the unknown text corpus of the long-term compressor) as well as estimate entropy via Y algorithms. You'll be wasting a lot of CPU on these tasks. Meanwhile, humans have no issue because while english does compress somewhat, when we express unique ideas it's less compressible then when we do not.