I'm betting there are more than a few sitting in collections that aren't going anywhere soon. Even HG copies. Genre collectors tend to keep their comics once acquired and aren't likely to feel a need to slab them.
If prices continue to rise, and I'm of the belief large price increases on horror have topped off for now, you may see many more come out. But I'm guessing the rise would have to be higher than your normal price increase for genre collectors to let them go. Especially those with HG copies as the effort to get them was probably more difficult than your average superhero comic.
Give it 5-10 years though when the cycle repeats itself and horror back issues re-ignite and you'll likely to see the Census numbers rise quickly.
If a crash happens in the meantime though, all bets are off...
Jim