Can I populate a custom Customer field in such a manner that will automatically populate all prior invoices retroactively?

You'll need to manually edit each past invoice to include the newly created custom field information, office322. Currently, QuickBooks doesn't support automatic retroactive updates of custom fields to existing invoices.

Moving forward, you can ensure that this custom field is automatically included in all new invoices for each customer by adding it to the customer's profile.

Here's how:

1. Navigate to the Sales menu and choose Customers.
   
2. Click on the name of the customer for whom you want to add custom field information.
   
3. Tap Edit.
   
4. Enter the information in the custom field you created, then click Save.
   
    

I've included this article to show how to personalize and add specific info to your sales forms: Customize invoices, estimates, and sales receipts in QuickBooks Online.

You can revisit this thread if you need further assistance with your QuickBooks matters.