Absolutely. In park restaurants such as Crystal Palace, Cinderella's Royal Table or Akershus Princess Breakfast in Epcot the earliest ADRs are typically one hour prior to park opening. You just show up at your park early and they will have a gate marked for those with breakfast reservations. They'll let you in according to your ADR time.
I believe the resort breakfasts begin even earlier than this. Ohana and Chef Mickey's start (I think) anywhere from 7:00 to 7:30.
BTW, we always get an early ADR for the Magic Kingdom on one of our days. The past couple of trips we've eaten at Crystal Palace and in the Castle at around 8:00 a.m. A couple of perks of this are:
-walking down Main Street with almost no one there. Pretty cool!
-first breakfasts mean the food is much fresher!
-we got done both times around 8:50, and were able to be right there at the Fantasyland rope drop at the castle tunnel at 9:00. While everyone else is coming through the turnstyles and fighting their way down Main Street, we were able to do everything in Fantasyland in about 1 hour!!!!!
Hope this helps!