Setting the draft.watermark.image parameter value

Hi all,
 
I'm trying to allow my users to set their own watermark simply by defining an image labeled "watermark" within their document. This means I need to set the value of the draft.watermark.image value dynamically. This is a little difficult because, as far as I can tell, the parameter value needs to be set before the target file is processed (makes sense, just inconvienent at the moment). Is there any boilerplate way of getting around this?
 
Thanks,
Jeff