I'm posting a follow up to provide some guidance to others in case they do the Saturday hours thing again.
The Saturday hours at the Miami passport agency were 9:00am to 3:00pm, with no appointments.
My wife and I got to the agency a little before 5:00am, and got in line, there were around 30 people ahead of us, eventually that turned into closer to 50, since some of them were holding the place for members of their group. There were a lot of families, from discussions with people nearby, it seemed that around 2/3 were getting passports, the rest were parents, family members or friends. The queueing area was in the parking garage, it was fairly hot and stuffy, but at least not in the sun. We had brought folding chairs, but the security guards said they were not allowed, so we weren't able to use them.
I talked to one of the people near the front of the line, they got there around midnight.
Here are estimates for the number of people in line at various times:
5:00am - 35-40
6:00am - 80-90
7:00am - 200-250
8:00am - 400-500
9:00am - around 1000 people and streaming in steadily.
Before 8:00, the only officials were building security, around 8:00 armed Dept. of State Diplomatic Security (DSS) agents took over. Announcements were made that this office was only for emergency passport issuance for people traveling within 14 calendar days, and that if you didn't have documentation to that effect, don't bother staying in line. Then the agents went through the line checking for documentation (not that creating and printing a fake airline itin would be too hard).
A little before 9:00, the agents went through again looking for people leaving on Monday, they were pulled out to be processed first. Then the queue was handled from the front, we were at the medal detector by 9:20, and completely done by 10:00. The passports are supposed to be overnighted on Tuesday for Wednesday delivery.
A few other random items:
The parking garage is $6 per hour, up to $30 max. A lot of people were walking into the garage, so they probably found cheaper parking elsewhere, on a Saturday there appeared to be lots of metered parking on the street available, but you would probably have to feed the meters every couple of hours.
I think the only on-the-spot issuance was for those traveling on Monday, otherwise they were Fedexing for about $15.00. The total price was $150 per person. $75 passport fee, $60 expedite, $15 Fedex shipping.
While waiting in line, I asked a couple of DSS agents how many people would get turned away, and both seemed to think that anyone who showed up by 3:00 would be processed. Based on the throughput in the office, that seemed unlikely to me, unless they were going to be open way past 3:00.
Lots of people were filling out the forms by hand in line. We did it on-line and printed them, that seemed to streamline the process once we got to the window.