Hello Andrew,
Your best option for imprinting information onto a pdf is to use the scripts which ship with PDFPen Pro or some modification of those.
This is what I used when scripting Bates numbering. This script might give you some ideas about how to go about it - Bates numbering of pdfs from within Devonthink
It should be possible using applescript to script the pdf capabilities of Smile’s satimage osax too, if you want a free option but you will need the help of someone with better knowledge of Smile. Might be worth asking on Macscripter.net
If you merely want to put a date into the name of the pdf that is much easier. Where is that date going to come from? Do you want to extract it from the document itself, take the creation or modification date of the file and place that in the name, or get the date from some third source such as a csv?
Frederiko