Umm...
Perhaps we need a new forum rule.
DO NOT... Post requests for help while suffering from severe flu symptoms
To clarify what I'm trying to achieve.
I start with a list of filenames in the following format:
<Author> - <Series_Name> <Series_Index> - <Title> (<Subtitle and/or Other Info>).<fmt>
This is split and formatted into the following:
Author (FN LN)
Author_Sort (LN, FN)
Series
Series_Index
Title
Subtitle
I then read the metadata from the source file, update it as appropriate with the above and call add_books() passing the updated metadata.
This works almost exactly as intended. The book is added with the correct metadata except for the missing custom column data.