This seems to preclude cases where pre-existing behaviors are co-opted as signals. Did you mean to preclude such cases?

Bleah. I notice that I am confused. Or at least, confusing. ;-)

What I was trying to say was that there's no reason to fake (or enhance) a characteristic or behavior until after it's being evaluated by others. So the evolutionary process is:

1. There's some difference between individuals that provides useful information
2. A detector evolves to exploit this information
3. Selection pressure causes faking of the signal

This process is also repeated in memetic form, as well as genetic form. People do a behavior for some reason, people learn to use it to evaluate, and then other people learn to game the signal.