For other attractions with separate wheelchair entrances, when we had Fastpass Plus, we were still given a return time ticket, but the start time on it was right away and we could get right into the accessible line.
Here is how Spaceship Earth worked before with a monility device -
If you approached the CM at the entrance to the regular line with a mobility device, that CM explained how to get to the accessible entrance and you were sent there.
The accessible entrance is at the end of the exit ramp from the attraction. One side has parking for mobility devices and the other side has a queue with some benches along the wall.
A CM at the area had a clipboard and wrote people's names and party size on it as they arrived. You were given an approximate waiting time. Most people just joined the queue at that point, but people sometimes left and came back.
We had a FASTPASS Plus scheduled for Spaceship Earth on our last trip in April 2014 during Spring Break and didn't use it. We had also been to WDW in October 2013, after
DAS began.
The times we went, only a few people were in the area waiting and the process seemed like they were doing the same as previously.
It's possible that if it had been busier, more people were waiting, or someone wanted to wait somewhere else, they would have used return time passes.