Everytime we've had any ADRs for MK before or around opening we were allowed in as soon as we tried to get in. I've never even been asked for a confirmation number, name or anything. I'm sure it depends a bit on the CM, but in all our early MK ADRs I've never been asked anything. And two of those times were Thanksgiving week, so I don't think the time of year really makes a difference. Good luck. Walking down an almost empty main street is special.
Just wanted to add that normally when making ADRs you are asked to arrive around 15 minutes prior to your reservation time. They'd need to let you in earlier than 9 for you to be able to make it to your ADR the 15 minutes early they ask for.