EDIT: Found it, shortly after posting, ignore the rest. 
The assignment is indirect as the status flags are enumerated and then displayed as buttons on the book status page, within the book.
I was just exploring the source to see how hard it would be to fix something that's been bugging me and I saw the "abandoned" status flag, with the associated "On hold" text, which led me to a google search that brought me here.
I can't seem to figure out how to "abandon" a book. I have one book that I want to abandon for now (got bored with it), but the only option I see when holding down on the book is "Mark as read", "Add to favorites", etc, but nothing to "abandon" it or put it on hold. That said, your plugin description above says that it only syncs from device to Calbire, so it must be something that can be set on the device, yet I can only find comparisons against the value, nothing that sets the value in the source making me think that it's an incomplete feature or one that has only been partially removed.