I agree with frono on John Denver. I could see his fans thinking about a Guild if they see him playing one, but no way he would influence their financial fortunes!
IMO, Richie Havens and Barry Gibb are really the only "relatively big" artists that actually played Guild almost exclusively in public. I would argue neither would affect their fortunes - Gibbs signature Guild would have to have been produced in much higher quantities! Others might have done an ad for them, like Clapton, but didn't really play them much. I'm not sure I would count Tommy Smothers as an artist of influence, or Doyle Dykes, or Zal Yanovsky for that matter. This is off the top of my head, feel free to correct me if I'm wrong.
The only artist that could have done this is if The Beatles had actually used the Guild 12-string that was given to Lennon on stage, or in some other public venue, or even in the studio. That would have given Guild some attention, although even that is questionable as to whether it would have affected their fortunes. Certainly Epiphone sold more guitars because of their use of the Casino. But once Dronge handed it John and not George, that ship sailed away. Much more likely George might have actually used it in public, or even in the studio - instead he looks annoyed!
walrus