I remember reading from a previous thread how yes you can do something like that (or lots of other techniques) but they will only work on a small percentage of files.

That sometimes the smallest decompressor (including the key, in your example) will result in overall less space, but usually it will result in more (because the smallest key that generates 10 zeros happens to be 14 digits long, etc.).

And so ultimately the more interesting question becomes about those probabilities, and therefore what is the right entry price for the bet to be even?