@Kris Wood You should use a custom list or record for the source

Currently the select field is sourced from 'customer'. They want to filter out customers that have a custom "third party billing" checkbox checked. I think what I'll need to do is not have a 'source' list but rather create an empty select field, then field.addSelectOption the results of a search that returns all the customer IDs that of customers who don't have that box checked.