And you're right: upon testing it seems that the drop-down selector is only allowing me to choose from text and datetime field types. Bummer, but I can make that work. I'll reconfigure my custom fields from radiobuttons to text.
Thanks for your explanation and +1 vote for a future release where any list field type can be updated from a campaign. In the event the user sets a campaign to auto-update a field with value not permitted per the list field setup (eg a non-existent radiobuttons value) those should just fail to update.