Custom printer: digital signature on last page?

Is it possible to create a custom printer which signs the document at the last page?

I have documents with arbitrary number of pages, but I want them with a visible digital signature at the last page. 

How can I accomplish that?

If not possible, I would like to sugget the Devs to enable negative numbers in the interface.
With no other changes to the interface necessary, negative values would count from last to first page. 
Therefore, -1 would be the last page, -2 the one before that, and so on. 

I would like to see the ability to work from the last page forward and agree that the negative numbers make sense. I need to put a watermark (letterhead) on the last page of a report but the number of pages in the report are unknown until sent to the printer.