Thanks for taking the time to dissect the downsides of what I'm increasingly convinced is a bad idea. I agree with your reasoning, particularly with regard to the scenarios involving hashtables and lists.
Nonetheless, variable negation is something I imagine we all encounter with relative frequency, what do you guys think of a negation symbol equivalent to "no." (i.e., ¬)?
Since "!" is used elsewhere, "^" strikes me as a natural choice, probably because I associate it with set complements for POSIX regular expressions.