Just spitballing, but I'd hazard a guess that your odds are better for an early check in at some of the larger and/or less-popular resorts. OKW, SSR, and Coronado Springs guests probably have a better shot of an early check-in than others.
And for those of you who think I'm slagging off on those three resorts, I'm not. I think they're lovely, I'd stay in any of 'em in a heartbeat, but, based on what I've read here and discussed with others, they don't seem to ignite the same level of passion from the same numbers of people all the other resorts do. Personally, now that I'm in DVC, I'm trying to work my way through all the other resorts (VWL & BWV are next), but I look forward to staying @ SSR again, especially once construction is completed.