It's not as simple as just printing off a supplier invoice. These are customer invoices issued on behalf of the supplier. - FA would effectively have to create a customer invoice without affecting AR or stock levels and then processing it as a normal supplier invoice. And the created invoices would have to be numbered on a per-supplier basis, presumably with a different prefix for each supplier.
I'm not saying it can't done, but there would have to be changes to the database structure. And while most of the code required probably already exists, there's the matter of fitting this thing into the current workflow and menus. We can probably expect users to distinguish between customer and supplier invoices, but here's something which could be confused with both. Should it be enabled by default? If not, should the setting be per company, user or supplier? There's a lot that need to be thought through if we want such a feature.
Anyway, this kind of invoicing isn't an accounting issue at all, but rather an administrative shortcut. While you do need an invoice to match the payment, the accountant (or accounting system) doesn't really care who issued it.
For now, I suggest you create an invoice in your favourite word processor and let FA handle it as any supplier invoice.