Hi Scott,
If you are having multiple charges in an invoice, I am assuming it would create multiple items of the FICA posting document. And given the fact that receivable GL account is a part of fkkop structure, you can make use of FQEVENT 1102.
This event is triggered during document posting and the structure E_FKKOP is exportable where you can set any value inside this event.
Please check if this event is triggered in invoicing document posting, if yes, you can implement here.
Thanks,
Bodhisattwa