ok I came up with the following:
added a custom column: 'formatted_series_digits'
and a custom column:
Code:
program:
finish_formatting(field('series_index'), strcat("0>", ifempty( field('#formatted_series_digits'), '1'), "s"), '', '')
kinda hacky in that you need to specify how many digits you want, but only a few series have > 10 books so I can live with it.