http://abr.business.gov.au will be good for determining the type of entity applying if you want to drop that question as well.

However, I assume most of the applications will be for a Company/Company+Trust. To get this data, if you do a company search via ASIC ($9 for the extended version), you can get this: https://connectonline.asic.gov.au/idc/groups/exportalcontent...

I am fairly certain nab will be doing a similar search so maybe you can work with them on that or have them reimburse you for Braintree performing the initial search.

Re. ABN/ACN there are certain (very rare) situations where the ABN and ACN differ apart from the additional 2 numbers at the start of the ABN.