I'd like to let users update their preferences/settigns by just clicking a link.
So I might ask "would you like to be a beta tester?" and have a link that when clicked updates that field.
I thought I saw that somewhere but can't find it now.
It sounds like you may be after the updating a subscriber method to which you can then update the custom field with the relevant data.
It may work in a similar way to tracking RSVPs as mentioned here - so when the user selects the "Yes" or "No" link, the script on your web page would check for the subscriber in your list, and update their details accordingly.