No when the invoice is saved 2 things happen - the value is set from the sales order to the invoice and the emails are sent out. I don't want to change how the email is sent out because this is for mass invoicing.

How is the email sent? Script/workflow? Native process? What entry point?

It's sent via native process with a script running on it to set email address values.

Is this through Invoice Billable Customers?

It's through Invoice Sales Orders. (The billing team needs to be able to see the memo).

We don't use this feature. Try setting the value Before Record Submit? Best guess.

And the value will "stick"?

If you set it before submit, it will be submitted, i.e. saved.

Great, thank you so much for your help!

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Thanks.