Voigt's done more for the search for Zodiac than anybody else!
Also on the special features DVD of the director's cut of Zodiac, Officer Richard Hoffman says that he never went to that phantom painting party and never knew Darlene Ferrin. Also on page 470 of Zodiac Unmasked Graysmith even says that Hoffman wasn't at the party, although he was on the guest list. And since there was of course never a party, there couldn't have been a guest list! Thus your theory falls apart!