Thanks for getting involved with this thread, econnolly.

I can certainly understand how an ability to select an income account for each line item while creating an invoice could be useful and have submitted a suggestion about it as of today.

You can also submit your own feedback while using QuickBooks.

Here's how:
 

1. In the top menu bar, go to Help, Send Feedback Online, then Product Suggestion....
   
   
    
2. Choose a Type of Feedback and Product Area (optional) from your available drop-down menus.
   
   
    
3. Enter any suggestions/feedback in the Here is my suggestion: field.
   
   
    
4. If you'd like, include your name/email in the My name is: (optional) and My e-mail address is: (optional) fields.
   
   
    
5. Select Send Feedback.
   
   

Your feedback's definitely valuable to Intuit. It will be reviewed by our Product Development team and considered in future updates. You can stay up-to-date with the latest news about your product by reviewing Intuit's Product Updates webpage.

For the time being, you can use the workarounds mentioned by my colleagues in this thread. You also have the option of checking our QuickBooks App Store for apps which may be able to help you achieve what you're looking to accomplish.

If there's any additional questions, I'm just a post away. Have a lovely Friday!