I had a sniff around the Embassy's website yesterday after making my post. They definitely add pages at no cost in the American Citizen Services office, located in the Annex across Wireless Road from the main Embassy Chancery. I also discovered the closed-last-Friday-of-the-month thing, which basically means I have only one chance on my upcoming visit.

But the website also allowed me to make a reservation, which I did. Hopefully that will make things go more smoothly than if I just showed up.