Hi Brandon,
I don't think there is any really reliable work around for this problem, so the SDK may indeed be your best bet.
The core problem appears to be when the invoice is created ? It should be created when the goods actually arrive at the customer correct? That way the sales are always booked in the correct month, and the customer does not complain that the invoice was received before the shipment.
I recommend building an "Invoice assistent". A program that for example looks at the due date of deliveries, or at a userfield in which you can fill in the realized receipt date.
A simple query that lists the deliveries that actually need to be invoiced, and then copies these deliveries to invoices.
We use this system, and not only does it enforce a whole lot of special scenarios, it also reduced the daily invoicing process to pretty much a single click.
Regards,
Johan