Thank you for reaching back to us, @aainc.

Your suggestions are important to us in developing the product. The feature to ​​automatically detect custom field data in the subject line of an email is still unavailable. We're unable to provide the exact time frame to when this will be added to our program. 

Our developers based the adding of features on the number of requests they received from customers like you. As always, we encourage you to visit the QuickBooks Blog to check for new updates and features that we have. 

You can refer to the following article for more information about personalizing and customizing the appearance of your sales forms in QuickBooks Online: Customize invoices, estimates, and sales receipts in QuickBooks Online.

Please feel free to visit us here on the Community page if you have any other invoices concerns, I'll be always here to help you. Have a great day ahead!