Generally no but it depends on when the CM's get around to taking the covers off of the fastpass machines.
At Magic Kingdom entire "lands" (Adventureland, etc.) might not be open during the EMH hour.
For some rides the fastpass machines might not be uncovered for awhile even after public hours start. This in turn depends on anticipated crowding at the park.
Disney hints:
http://members.aol.com/ajaynejr/disney.htm