I found the solution in a post on FlyerTalk, and I'll include it here in case it might help someone:
Select a Ding offer and move forward with either a round trip or one-way booking. Then, back out by clicking "PLAN TRIP" and book the one-way in reverse, or even take advantage of offers not in your ten-city list.
Using this method, I was able to book my one-way flight from Ontario to Houston even though Houston didn't show up when I looked under Dings for Ontario. And the trip to Ontario is for
Disneyland, so it should be considered on topic on this forum.
This makes me wonder if I've missed some Ding opportunities since they apparently don't list all the city-pair combinations that are available! For example, if I only had Ontario listed as one of my cities, I wouldn't have known that Houston to/from Ontario was included, and wouldn't have tried to book it. This seems to open up possibilites of "hidden" Dings! This is crazy - now I feel like I have to try the method above for every Ding offer and see if one of the cities I need is "hidden".
Does this make sense to anyone? Has anyone else seen this happen? I'm going to try to change the subject of my original post to include "hidden Ding offers" and see if that gets anyone's attention.