There's a clip of HBO's main campaign from 1983 ("There's No Place Like HBO") on YouTube that has some footage that probably would count as an original promo; about halfway through it we get a shot of several children watching it on TV (it's a little hard to make out from the distance the set is from our viewpoint in the shot, but you can see Gobo and another character I can't positively identify walking down the tunnel from Doc's workshop), followed by the characters' identification shot from the opening run backwards ("We're Gobo, Mokey, Wembley, Boober, Red."--but for this shot we go from Red to Gobo). It should be on either the first or second page if you type HBO into the search bar on top of the page.