Harry Potter fanfiction and its readers is way closer to mainstream culture than the sort of people who could read that crossover fic; keep in mind, Harry Potter fanfiction novels have been published as have entire books just predicting what would happen in book 7 or comprising dictionaries of Harry Potter-ania (you may remember the lawsuit over the latter).
I had one in my comment—that respectability varies more between mainstream culture and the people who are likely to read HPMOR than you realized.
Harry Potter fanfiction and its readers is way closer to mainstream culture than the sort of people who could read that crossover fic; keep in mind, Harry Potter fanfiction novels have been published as have entire books just predicting what would happen in book 7 or comprising dictionaries of Harry Potter-ania (you may remember the lawsuit over the latter).