Except - and here's a nice idea - let's say all the mailing lists standardize on a "We request your authorization" message, right? Most mailing lists request that anyway. But, here's the trick - when you agree, when you *reply* to the mail, it catches your reply going out, parses the header info out of it, and adds that to a whitelist. Not when the authorization message is sent to you, note, when you reply to *it*.
Wonder if anyone will do that.