Despite being a child of the nineties, and thus having been overexposed to the Oasis song of the same name, I still had to look up the meaning of the word "wonderwall" after reading this story. Thanks to the Urban Dictionary, I now know that it roughly means "someone you find yourself thinking about all the time, or are infatuated with." Apparently, it is also the name of a German Pop Band, and a solo album by Ringo Starr, which, in turn, was the soundtrack to a 1968 movie of the same name. Thanks, Internet!
Getting back to the story itself, if we take the above definition of "wonderwall" to be the one applied here (which I am, since I havn't been able to find another definition anywhere), there seems to be two possibilities for the "wonderwall" of the story itself. The most obvious seems to be to be the narrator's friend, David Baldanders, since the story itself starts with the statement "A long time ago, nearly thirty years now, I had a friend who was waiting to be discovered. His name was David Baldanders. . .," and, in turn, ends with her meeting him several years later.
However, while the narrator's relationship with David sets the framework for the story, I'd personally propose that the "wonderwall" of the title is actually the nameless, skinny, lank-haired, blond boy that keeps appearing to the narrator, calling her a "poseur." I think this apparition, lets call him "poseur-boy," becomes the true object of obsession for the narrator in the story, as we witness her descent into a spiral of heavy drinking and alcohol abuse in the pursuit of what she sees as "High Art." In fact, I would argue that "poseur-boy" is a sort of physical manifestation of art, and that, through his violence towards her (he attacks her at one point in addition to shouting "poseur" at her every time she sees him), is trying to show her that the life she's living is not the way to get to a place to create art. He his trying to show her that, in the folly of her youth, she has perhaps mistaken the drug and alcohol abuse of her idols as fuel for their artistic brilliance, when they were, in fact, secondary to sid brilliance. More importantly, I believe the manifestation of "poseur-boy" it trying to show her that, while that life may have "worked" for others, it is only destroying her.
That's my take on it, at least. Maybe I'm wrong.