Distrustful,

The reason that most of our efforts are geared toward law enforcement/military/intelligence positions is because these employees/applicants comprise the majority of people who can still be "tested" under current US law.  Nearly everyone else (employees of private firms, criminal suspects, etc) already has the right to refuse to be polygraphed and suffer no adverse consequences on the basis of this refusal alone.

Nonetheless, feel free to e-mail me any specific ideas you might have for such a letter (or better yet, a complete model letter).  We will be happy to consider it for posting on the site.