Thanks for getting back to this thread, @allopart.

Adding a custom fields to your expense invoices is unavailable in QuickBooks Online (QBO). For now, you can only add customary fields in your sales transactions and purchase orders. 

I can see how this option would be helpful for you and your business. Therefore, let me take note of this as a suggestion to improve your QuickBooks experience.

In the meantime, I recommend visiting our Firm of the Future site. This is where we share recent happenings and future developments, such as updates to newly added features.

Let me know if you need further assistance with this. I'm always around to provide answers and clarifications to your questions. Have a great day ahead and take care.