I've never watched the show, but Conan O'Brien randomly showed up in the background of an episode of How I Met Your Mother. He bought a walk on role in a charity auction. The writers wanted to give him a speaking role but he thought it would be funnier if he was just randomly in the background.
The Walking Dead has had giveaways like this, haven't they, where some random winner gets a bit role as a zombie? They could somehow do the same thing with a random celebrity if they wanted to. Having Rick and co. recognizing someone, to me, feels like it would cheapen the show a bit. The vast majority of these walkers are so rotted away at this point that they should be recognizable anymore.