I’m interested in a custom field for toggling a single, specific taxonomy term relationship on/off.
In the settings for the field, the user would be able to choose the term, and the input type (appearance).
So similar to a true/false or a simple checkbox, but instead of saving a meta value, it would set/remove the specific term relationship.
I will bring this to the attention of the developer. I can’t say if it’s something he’d want to add or not.
You might want to look into perhaps creating an add on
or using some of the available filters. For example, this could probably be done using a true false field and the acf/save_post action http://www.advancedcustomfields.com/resources/acfsave_post/