Have you read Move Under Ground by Nick Mamatas? That's my favorite piece of Beat RPF not by an actual Beat--it's a Kerouac/Lovecraft mashup that starts with Jack seeing the drowned city of R'lyeh rising out of the sea at Big Sur and just gets more twisted and brilliant from there. OK, it's a book, not a fic as we think of it, I suppose--but it TOTALLY is! And a crossover/AU at that.