I figured it out. Super simple. Duplicated the members app, added a static function to check the ID and added the helper to the form template. Once I figured out how helpers work in the context of form validation, it was quite straightforward.
I have a form that will have a "Company ID" field that needs to be validated against a database of existing company ids. Functionally, it will work similar to the existing email check, though it will come from a separate table in the database.