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. 

To ensure that the custom field automatically appears on all new invoices for a specific customer, let's add it to the customer's profile. Please note that this will only apply to future transactions and will not update past invoices.  

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.