Last June we had a 8:10 Crystal Palace reservation. We were out by 8:50 and walked over to the rope blocking off Tomorrowland. A cast memeber asked us what we were planning to do first, and we told him Space Mountain. He let us go past the rope and told us if someone was out in front of Space Mountain and would let us, we could go ahead and ride it. There was, and we did. It was so cool being the only people on the ride. We were off right at 9 am and then headed out to the rest of the park.
I didn't get the impression this was a common thing, but I would say it is worth seeing if you can make your way to Space Mountain first to get that ride in, and then you could make it over to 7DMT walking through Tomorrowland and still beat the big crowds.