Take a look at this project:
PDFCreator[
^].
or this:
EmfPrinter[
^]
Probably a serious amount of work, but it should get you there ...
From your description I can't see any really simple and robust solution.
You'll find source code for a printer device driver as a part of the
Windows Device Driver Kit[
^]. If you create a virtual printer driver, you will have full control and know exactly what your users are sending to the printer.
Regards
Espen Harlinn