Martyn,
AM General has a separate test operation in Michigan and they were kind enough to share some of their findings with us. They actually put our tents through a series of tests, including their "sled" or crash test. The ladder was surprising to us how strong they were, they tested the tall alloy ladder for the Maggiolina, and as Mike described, it is a breaking or failure strength. The reality is that the tall alloy ladder for the Maggiolina and Columbus will flex and bend at about 250 pounds, especially at an extreme angle. The standard height ladder, the one you use on your trailers, is effectively stronger as the span isn't as great.
The ladders for the OverLand series and the OverCamp are different and stronger as these ladders acts as a compression struts as well as a ladder. You may have someone inside the tent as well as someone climbing up so the potential load could be greater.
My pet peeve about the ladder design on folders, as Mario knows, is that I consider many ladders to be a problem. They are designed for expedition use, namely camping in the rough. When you camp in a park, on the hard, the base of the ladder can potentially slide on a hard slick surface. Even if the ladder is strong, if it slides, it adds little support, the ladders really need to incorporate a idiotproof locking mechanism if they are sold to the general public. Martyn, this isn't really an issue for you at AT because you can engineer the tents to fit your trailer. I know from working with AutoHome that it isn't so easy for them as they don't know the application the tent will be used in and have to plan accordingly, and thus have locking mechanisms on all their folders they sell.
Bottom line, it isn't so much about the physical strength of the ladder, it is how well the ladder works with the tent that determines how safe it is.
I know, too much information, but your point about using the ladders as sand ladders is well taken. If you damage one of these ladders, you can repair them in the field without welding. Some of the old steel expedition ladders used to come back looking really bad from being driven over, the alloy ladders are a big improvement but cost about four times as much.
Rich Haslacher