Does anyone know of an option or plugin which will allow Safari to accept third party cookies from particular "trusted" web sites?

Or maybe something that gives Safari support for P3P?

Background:
For those who don't know, P3P is an old and little-used W3C standard which allows companies to publish how they promise they will/won't use web visitor data, in exchange for letting that site set third party cookies. Most browsers besides IE don't support it, or are dropping support for it.

That said, since one of the sites I work requires third party cookies for legitimate reasons, I'm looking for an option for using Safari without clicking "Accept all cookies."