Digital Signature on PDF Invoice

Our company needs to add a digitial signature to the PDF invoice that goes to customers. This is a local law in one of the country we are rolling PeopleSoft Billing. We are using BI Publisher templates to produce PDF copy of the invoices. We are on PeopleSoft Finance 9.1 and Tools 8.52.11. Our team has found that the PeopleSoft 9.2 supports this feature, however we cannot upgrade at the moment.

This is a critical requirement. Is there a way or a work around to achieve this through existing application/tools version? Any recommendation/advise is greatly appreciated. Thank you