I can help you with your invoice,

This is a fantastic thought, @aainc, and I can see how this would be very useful.

At this time, however, this option is not available to map custom fields to the subject lines within an invoice. 

I’m happy to share this feedback with our development team for future changes within the product, and you can always keep an eye on our blog for the newest updates within our Intuit products.

If you find it helpful, I've included this article for you to look over: How to Customize Invoices in QuickBooks Online.

Please let me know what else I can help you with, I’m only a post away to answer any questions! Have a fantastic weekend.