Well, here's the thing -- both things happen. The locals rush out and a lot of people wait for the MHP.
Also, weather can play a role. For example, about 5 years ago, there was a horrible heat wave leading up to the start of Halloween Time. We're talking 118 degrees at
Disneyland, and 113-115 degrees in Los Angeles. It was miserable. There are a lot of people -- like myself -- who would rather not be at Disneyland in that kind of heat, so they stay away until it cools down a wee bit.
Also, 8 years ago, I was at DL over the opening of Halloween Time and it was intermittently pouring rain. It was the strangest weather I've ever experienced in Disneyland and DCA, because it would absolutely pour buckets (not a light sprinkle), causing everyone to run for ponchos and flee to the indoors, and then would get sunny and hot -- to the point where we were peeling off the ponchos and getting out the sunscreen. Then it would start pouring again, and then get hot again.
Any kind of weird weather or extreme weather like what I mentioned above can keep crowds away.
On a Halloween Time opening day when it was 'normal' weather -- which was maybe 77 degrees, overcast and humid -- I noticed that the first part of the day was quite pleasant, with lighter crowds. Later in the day, things picked up as people got out of work and school.
Anytime a season like Halloween Time or the Holidays officially opens, a lot of people will flock to DLR to see what's new. These days there are so many bloggers, podcasters, moderators, reporters, etc., all involved with Disney sites, and they want to get there while everything is newsworthy. No one really wants to wait 2 more weeks to go to Disneyland and see everything after everyone else has seen it -- especially when they have to write about it or report about it.
But... then Mickey's Halloween Party rolls along 2 weeks later, and all of those people who rushed to Disneyland to be there on the first day or weekend of Halloween Time will rush back to be there for the first night of MHP (again, it's newsworthy). Along with them will come all the people who were holding off to visit DLR until MHP begins (like the people who are tourists and more locals).
So, because Halloween Time starts on 9/11, unless there is some sort of weird or extreme weather event to keep people away, I would expect there to be decent crowds -- especially later in the day -- especially if anything new is added in to the roster of seasonal activities. It's not going to be madness like New Year's Eve, or post-Christmas, crowds... but it will be busy because people love Haunted Mansion Holiday, Ghost Galaxy and Halloween Time in general.