So far, I believe I have worked out how to set a bookbag/list to be a patron's default list. When a logged in patron accesses their lists in TPac for the first time, they will see this:
You can see above that the patron in question, me, has no default list chosen. After clicking the "Make Default List" button next to the "My List" entry, the patron's list screen will look like this:
I know you are probably thinking, "That's it? What has he been doing for the past couple weeks?" "Working on other things," is the answer. I was hoping to get more time to work on this, but other things have come up. It should be done real soon, now, on the order of a week or two depending on how much free time I can steal at home.